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                                                                    Exhibit 4.6

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                    PREFERRED SECURITIES GUARANTEE AGREEMENT

                                       by

                           W. R. BERKLEY CORPORATION,
                                  as Guarantor

                                   relating to

                         W. R. BERKLEY CAPITAL TRUST II

                            Dated as of July 26, 2005

================================================================================

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                                TABLE OF CONTENTS


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ARTICLE I.      DEFINITIONS AND INTERPRETATION..........................................    1

Section 1.1.    Definitions and Interpretation..........................................    1

ARTICLE II.     TRUST INDENTURE ACT.....................................................    4

Section 2.1.    Trust Indenture Act; Application........................................    4
Section 2.2.    Lists of Holders of Securities..........................................    4
Section 2.3.    Reports by the Preferred Guarantee Trustee..............................    5
Section 2.4.    Periodic Reports to Preferred Guarantee Trustee.........................    5
Section 2.5.    Evidence of Compliance with Conditions Precedent........................    5
Section 2.6.    Events of Default; Waiver...............................................    5
Section 2.7.    Event of Default; Notice................................................    5
Section 2.8.    Conflicting Interests...................................................    6

ARTICLE III.    POWERS, DUTIES AND RIGHTS OF PREFERRED GUARANTEE TRUSTEE................    6

Section 3.1.    Powers and Duties of the Preferred Guarantee Trustee....................    6
Section 3.2.    Certain Rights of Preferred Guarantee Trustee...........................    8
Section 3.3.    Not Responsible for Recitals or Issuance of Guarantee...................    9

ARTICLE IV.     PREFERRED GUARANTEE TRUSTEE.............................................   10

Section 4.1.    Preferred Guarantee Trustee; Eligibility................................   10
Section 4.2.    Appointment, Removal and Resignation of Preferred Guarantee Trustees....   10

ARTICLE V.      GUARANTEE...............................................................   11

Section 5.1.    Guarantee...............................................................   11
Section 5.2.    Waiver of Notice and Demand.............................................   11
Section 5.3.    Obligations Not Affected................................................   11
Section 5.4.    Rights of Holders.......................................................   12
Section 5.5.    Guarantee of Payment....................................................   13
Section 5.6.    Subrogation.............................................................   13
Section 5.7.    Independent Obligations.................................................   13
Section 5.8.    Taxes...................................................................   13

ARTICLE VI.     LIMITATION OF TRANSACTIONS; SUBORDINATION...............................   13

Section 6.1.    Limitation of Transactions..............................................   13
Section 6.2.    Subordinated Ranking....................................................   14

ARTICLE VII.    TERMINATION.............................................................   14

Section 7.1.    Termination.............................................................   14

ARTICLE VIII.   INDEMNIFICATION.........................................................   15
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Section 8.1.    Exculpation.............................................................   15
Section 8.2.    Indemnification.........................................................   15

ARTICLE IX.     MISCELLANEOUS...........................................................   16

Section 9.1.    Successors and Assigns..................................................   16
Section 9.2.    Amendments..............................................................   16
Section 9.3.    Notices.................................................................   16
Section 9.4.    Benefit.................................................................   17
Section 9.5.    Governing Law...........................................................   17
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                    PREFERRED SECURITIES GUARANTEE AGREEMENT

      This GUARANTEE AGREEMENT (the "Preferred Securities Guarantee"), dated as
of July 26, 2005, is executed and delivered by W. R. Berkley Corporation, a
Delaware corporation (the "Guarantor"), and The Bank of New York, a New York
corporation, as trustee (the "Preferred Guarantee Trustee"), for the benefit of
the Holders (as defined herein) from time to time of the Preferred Securities
(as defined herein) of W. R. Berkley Capital Trust II, a Delaware statutory
trust (the "Issuer").

      WHEREAS, pursuant to an Amended and Restated Trust Agreement (the "Trust
Agreement"), dated as of July 26, 2005, among the trustees of the Issuer named
therein, the Guarantor, as sponsor, and the holders from time to time of
undivided beneficial interests in the assets of the Issuer, the Issuer is
issuing on the date hereof 10,000,000 preferred securities, having an aggregate
liquidation amount of $250,000,000, designated the 6.750% Trust Originated
Preferred Securities (the "Preferred Securities") and four common securities,
having an aggregate liquidation amount of $100 designated the 6.750% Trust
Originated Common Securities (the "Common Securities");

      WHEREAS, as incentive for the Holders to purchase the Preferred
Securities, the Guarantor desires irrevocably and unconditionally to agree, to
the extent set forth in this Preferred Securities Guarantee, to pay to the
Holders of the Preferred Securities the Guarantee Payments (as defined herein)
and to make certain other payments on the terms and conditions set forth herein;
and

      WHEREAS, the Guarantor is also executing and delivering a guarantee
agreement (the "Common Securities Guarantee") in substantially identical terms
to this Preferred Securities Guarantee for the benefit of the holders of the
Common Securities (as defined herein), except that if an Event of Default (as
defined in the Indenture), has occurred and is continuing, the rights of holders
of the Common Securities to receive Guarantee Payments under the Common
Securities Guarantee are subordinated to the rights of Holders of Preferred
Securities to receive Guarantee Payments under this Preferred Securities
Guarantee.

      NOW, THEREFORE, in consideration of the purchase by each Holder of
Preferred Securities, which purchase the Guarantor hereby agrees shall benefit
the Guarantor, the Guarantor executes and delivers this Preferred Securities
Guarantee for the benefit of the Holders.

                                   ARTICLE I.
                         DEFINITIONS AND INTERPRETATION

      Section 1.1. Definitions and Interpretation. In this Preferred Securities
Guarantee, unless the context otherwise requires:

            (a) Capitalized terms used in this Preferred Securities Guarantee
      but not defined in the preamble above have the respective meanings
      assigned to them in this Section 1.1;

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            (b) a term defined anywhere in this Preferred Securities Guarantee
      has the same meaning throughout;

            (c) all references to "the Preferred Securities Guarantee" or "this
      Preferred Securities Guarantee" are to this Preferred Securities Guarantee
      as modified, supplemented or amended from time to time;

            (d) all references in this Preferred Securities Guarantee to
      Articles and Sections are to Articles and Sections of this Preferred
      Securities Guarantee, unless otherwise specified;

            (e) a term defined in the Trust Indenture Act has the same meaning
      when used in this Preferred Securities Guarantee, unless otherwise defined
      in this Preferred Securities Guarantee or unless the context otherwise
      requires; and

            (f) a reference to the singular includes the plural and vice versa.

      "Affiliate" has the same meaning as given to that term in Rule 405 of the
Securities Act of 1933, as amended, or any successor rule thereunder.

      "Authorized Officer" of a Person means any Person that is authorized to
bind such Person.

      "Business Day" means any day other than a day on which banking
institutions in New York, New York or Wilmington, Delaware, are authorized or
required by law or executive order to close.

      "Common Securities" means the securities representing common undivided
beneficial interests in the assets of the Issuer.

      "Corporate Trust Office" means the office of the Preferred Guarantee
Trustee at which the corporate trust business of the Preferred Guarantee Trustee
shall, at any particular time, be principally administered, which office at the
date of execution of this Agreement is located at The Bank of New York, 101
Barclay Street, 8W, New York, New York 10286, Attention: Geovanni Barris.

      "Covered Person" means any Holder or beneficial owner of Preferred
Securities.

      "Debentures" means the series of junior subordinated debt securities of
the Guarantor designated the 6.750% Subordinated Debentures due 2045, held by
the Property Trustee (as defined in the Trust Agreement) of the Issuer.

      "Event of Default" means a default by the Guarantor on any of its payment
or other obligations under this Preferred Securities Guarantee.

      "Guarantee Payments" means the following payments or distributions,
without duplication, with respect to the Preferred Securities, to the extent not
paid or made by the Issuer: (i) any accrued and unpaid Distributions (as defined
in the Trust Agreement) that are required to

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be paid on such Preferred Securities to the extent the Issuer shall have funds
available therefor at such time, (ii) the redemption price, including all
accrued and unpaid Distributions to the date of redemption (the "Redemption
Price") to the extent the Issuer has funds available therefor at such time, with
respect to any Preferred Securities called for redemption, and (iii) upon a
voluntary or involuntary dissolution, winding-up or termination of the Issuer
(other than in connection with the distribution of Debentures to the Holders),
the lesser of (a) the aggregate of the liquidation amount and all accrued and
unpaid Distributions on the Preferred Securities to the date of payment, to the
extent the Issuer shall have funds available therefor at such time, and (b) the
amount of assets of the Issuer remaining available at such time for distribution
to Holders in liquidation of the Issuer (in either case, the "Liquidation
Distribution"). If an event of default under the Indenture has occurred and is
continuing, the rights of holders of the Common Securities to receive payments
under the Common Securities Guarantee Agreement are subordinated to the rights
of Holders of Preferred Securities to receive Guarantee Payments.

      "Holder" shall mean any holder, as registered on the books and records of
the Issuer of any Preferred Securities; provided, however, that, in determining
whether the holders of the requisite percentage of Preferred Securities have
given any request, notice, consent or waiver hereunder, "Holder" shall not
include the Guarantor or any Affiliate of the Guarantor.

      "Indemnified Person" means the Preferred Guarantee Trustee, any Affiliate
of the Preferred Guarantee Trustee, or any officers, directors, shareholders,
members, partners, employees, representatives, nominees, custodians or agents of
the Preferred Guarantee Trustee.

      "Indenture" means the Subordinated Indenture dated as of July 26, 2005,
among the Guarantor (the "Debenture Issuer") and The Bank of New York, as
trustee, and any indenture supplemental thereto pursuant to which certain
subordinated debt securities of the Debenture Issuer are to be issued to the
Property Trustee of the Issuer.

      "Majority in liquidation amount of the Securities" means, except as
provided by the Trust Indenture Act, a vote by Holder(s) of Preferred
Securities, voting separately as a class, of more than 50% of the liquidation
amount (including the stated amount that would be paid on redemption,
liquidation or otherwise, plus accrued and unpaid Distributions to the date upon
which the voting percentages are determined) of all Preferred Securities.

      "Officers' Certificate" means, with respect to any Person, a certificate
signed by two Authorized Officers of such Person. Any Officers' Certificate
delivered with respect to compliance with a condition or covenant provided for
in this Preferred Securities Guarantee shall include:

            (a) a statement that each officer signing the Officers' Certificate
      has read the covenant or condition and the definition relating thereto;

            (b) a brief statement of the nature and scope of the examination or
      investigation undertaken by each officer in rendering the Officers'
      Certificate;

            (c) a statement that each such officer has made such examination or
      investigation as, in such officer's opinion, is necessary to enable such
      officer to express

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      an informed opinion as to whether or not such covenant or condition has
      been complied with; and

            (d) a statement as to whether, in the opinion of each such officer,
      such condition or covenant has been complied with.

      "Person" means a legal person, including any individual, corporation,
estate, partnership, joint venture, association, joint stock company, limited
liability company, trust, unincorporated association, or government or any
agency or political subdivision thereof, or any other entity of whatever nature.

      "Preferred Guarantee Trustee" means The Bank of New York until a Successor
Preferred Guarantee Trustee has been appointed and has accepted such appointment
pursuant to the terms of this Preferred Securities Guarantee and thereafter
means each such Successor Preferred Guarantee Trustee.

      "Responsible Officer" means, with respect to the Preferred Guarantee
Trustee, any officer within the Corporate Trust Office of the Preferred
Guarantee Trustee, including any vice-president, any assistant vice-president,
any assistant secretary, the treasurer, any assistant treasurer or other officer
of the Corporate Trust Office of the Preferred Guarantee Trustee customarily
performing functions similar to those performed by any of the above designated
officers and also means, with respect to a particular corporate trust matter,
any other officer to whom such matter is referred because of that officer's
knowledge of and familiarity with the particular subject.

      "Successor Preferred Guarantee Trustee" means a successor Preferred
Guarantee Trustee possessing the qualifications to act as Preferred Guarantee
Trustee under Section 4.1.

      "Trust Indenture Act" means the Trust Indenture Act of 1939, as amended.

                                  ARTICLE II.

                               TRUST INDENTURE ACT

      Section 2.1.Trust Indenture Act; Application.

            (a) This Preferred Securities Guarantee is subject to the provisions
      of the Trust Indenture Act that are required to be part of this Preferred
      Securities Guarantee and shall, to the extent applicable, be governed by
      such provisions; and

            (b) if and to the extent that any provision of this Preferred
      Securities Guarantee limits, qualifies or conflicts with the duties
      imposed by Section 310 to 317, inclusive, of the Trust Indenture Act, such
      imposed duties shall control.

      Section 2.2. Lists of Holders of Securities.

            (a) The Guarantor shall provide the Preferred Guarantee Trustee with
      a list, in such form as the Preferred Guarantee Trustee may reasonably
      require, of the names and

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      addresses of the Holders of the Preferred Securities ("List of Holders")
      as of such date, (i) concurrently with the provision of such a list to the
      Property Trustee pursuant to the Trust Agreement, and (ii) at any time
      within 30 days of receipt by the Guarantor of a written request for a List
      of Holders as of a date no more than 14 days before such List of Holders
      is given to the Preferred Guarantee Trustee provided, that the Guarantor
      shall not be obligated to provide such List of Holders at any time the
      List of Holders does not differ from the most recent List of Holders given
      to the Preferred Guarantee Trustee by the Guarantor. The Preferred
      Guarantee Trustee may destroy any List of Holders previously given to it
      on receipt of a new List of Holders.

            (b) The Preferred Guarantee Trustee shall comply with its
      obligations under Section 311(a), 311(b) and Section 312(b) of the Trust
      Indenture Act.

      Section 2.3.Reports by the Preferred Guarantee Trustee. Within 60 days
after May 15 of each year, the Preferred Guarantee Trustee shall provide to the
Holders of the Preferred Securities such reports as are required by Section 313
of the Trust Indenture Act, if any, in the form and in the manner provided by
Section 313 of the Trust Indenture Act. The Preferred Guarantee Trustee shall
also comply with the requirements of Section 313(d) of the Trust Indenture Act.

      Section 2.4.Periodic Reports to Preferred Guarantee Trustee. The Guarantor
shall provide to the Preferred Guarantee Trustee such documents, reports and
information as required by Section 314 (if any) and the compliance certificate
required by Section 314 of the Trust Indenture Act in the form, in the manner
and at the times required by Section 314 of the Trust Indenture Act.

      Section 2.5.Evidence of Compliance with Conditions Precedent. The
Guarantor shall provide to the Preferred Guarantee Trustee such evidence of
compliance with any conditions precedent, if any, provided for in this Preferred
Securities Guarantee that relate to any of the matters set forth in Section
314(c) of the Trust Indenture Act. Any certificate or opinion required to be
given by an officer pursuant to Section 314(c)(1) may be given in the form of an
Officers' Certificate.

      Section 2.6.Events of Default; Waiver. The Holders of a Majority in
liquidation amount of Preferred Securities may, by vote, on behalf of the
Holders of all of the Preferred Securities, waive any past Event of Default and
its consequences except as provided for in Section 2.6 of the Trust Agreement
and Section 5.13 of the Indenture. Upon such waiver, any such Event of Default
shall cease to exist, and any Event of Default arising therefrom shall be deemed
to have been cured, for every purpose of this Preferred Securities Guarantee,
but no such waiver shall extend to any subsequent or other default or Event of
Default or impair any right consequent thereon.

      Section 2.7.Event of Default; Notice.

            (a) The Preferred Guarantee Trustee shall, within five (5) Business
      Days after the occurrence of an Event of Default, transmit by mail, first
      class postage prepaid, to the Holders of the Preferred Securities, notices
      of all Events of Default actually known to a

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      Responsible Officer of the Preferred Guarantee Trustee, unless such
      defaults have been cured before the giving of such notice, provided, that,
      the Preferred Guarantee Trustee shall be protected in withholding such
      notice if and so long as a Responsible Officer of the Preferred Guarantee
      Trustee in good faith determines that the withholding of such notice is in
      the interests of the Holders of the Preferred Securities.

            (b) The Preferred Guarantee Trustee shall not be deemed to have
      knowledge of any Event of Default unless the Preferred Guarantee Trustee
      shall have received written notice of such Event of Default, or unless a
      Responsible Officer of the Preferred Guarantee Trustee charged with the
      administration of the Trust Agreement shall have obtained actual knowledge
      thereof.

      Section 2.8.Conflicting Interests. The Trust Agreement shall be deemed to
be specifically described in this Preferred Securities Guarantee for the
purposes of clause (i) of the first proviso contained in Section 310(b) of the
Trust Indenture Act.

                                  ARTICLE III.

                          POWERS, DUTIES AND RIGHTS OF
                           PREFERRED GUARANTEE TRUSTEE

      Section 3.1. Powers and Duties of the Preferred Guarantee Trustee.

            (a) This Preferred Securities Guarantee shall be held by the
      Preferred Guarantee Trustee for the benefit of the Holders of the
      Preferred Securities, and the Preferred Guarantee Trustee shall not
      transfer this Preferred Securities Guarantee to any Person except a Holder
      of Preferred Securities exercising his or her rights pursuant to Section
      5.4(b) or to a Successor Preferred Guarantee Trustee on acceptance by such
      Successor Preferred Guarantee Trustee of its appointment to act as
      Successor Preferred Guarantee Trustee. The right, title and interest of
      the Preferred Guarantee Trustee shall automatically vest in any Successor
      Preferred Guarantee Trustee, and such vesting and cessation of title shall
      be effective whether or not conveyancing documents have been executed and
      delivered pursuant to the appointment of such Successor Preferred
      Guarantee Trustee.

            (b) If an Event of Default actually known to a Responsible Officer
      of the Preferred Guarantee Trustee has occurred and is continuing, the
      Preferred Guarantee Trustee shall enforce this Preferred Securities
      Guarantee for the benefit of the Holders of the Preferred Securities.

            (c) The Preferred Guarantee Trustee, before the occurrence of any
      Event of Default and after the curing or waiver of all Events of Default
      that may have occurred, shall undertake to perform only such duties as are
      specifically set forth in this Preferred Securities Guarantee, and no
      implied covenants or obligations shall be read into this Preferred
      Securities Guarantee against the Preferred Guarantee Trustee. In case an
      Event of Default has occurred (that has not been cured or waived pursuant
      to Section 2.6) and is actually known to a Responsible Officer of the
      Preferred Guarantee Trustee, the Preferred

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      Guarantee Trustee shall exercise such of the rights and powers vested in
      it by this Preferred Securities Guarantee, and shall use the same degree
      of care and skill in its exercise thereof, as a prudent person would
      exercise or use under the circumstances in the conduct of his or her own
      affairs.

            (d) No provision of this Preferred Securities Guarantee shall be
      construed to relieve the Preferred Guarantee Trustee from liability for
      its own negligent action, its own negligent failure to act, or its own
      willful misconduct, except that:

                  (i) prior to the occurrence of any Event of Default and after
            the curing or waiving of all such Events of Default that may have
            occurred:

                        (A) the duties and obligations of the Preferred
                  Guarantee Trustee shall be determined solely by the express
                  provisions of this Preferred Securities Guarantee, and the
                  Preferred Guarantee Trustee shall not be liable except for the
                  performance of such duties and obligations as are specifically
                  set forth in this Preferred Securities Guarantee, and no
                  implied covenants or obligations shall be read into this
                  Preferred Securities Guarantee against the Preferred Guarantee
                  Trustee; and

                        (B) in the absence of bad faith on the part of the
                  Preferred Guarantee Trustee, the Preferred Guarantee Trustee
                  may conclusively rely, as to the truth of the statements and
                  the correctness of the opinions expressed therein, upon any
                  certificates or opinions furnished to the Preferred Guarantee
                  Trustee and conforming to the requirements of this Preferred
                  Securities Guarantee; but in the case of any such certificates
                  or opinions that by any provision hereof are specifically
                  required to be furnished to the Preferred Guarantee Trustee,
                  the Preferred Guarantee Trustee shall be under a duty to
                  examine the same to determine whether or not they conform to
                  the requirements of this Preferred Securities Guarantee;

                  (ii) the Preferred Guarantee Trustee shall not be liable for
            any error of judgment made in good faith by a Responsible Officer of
            the Preferred Guarantee Trustee, unless it shall be proved that the
            Preferred Guarantee Trustee was negligent in ascertaining the
            pertinent facts upon which such judgment was made;

                  (iii) the Preferred Guarantee Trustee shall not be liable with
            respect to any action taken or omitted to be taken by it in good
            faith in accordance with the direction of the Holders of not less
            than a Majority in liquidation amount of the Preferred Securities
            relating to the time, method and place of conducting any proceeding
            for any remedy available to the Preferred Guarantee Trustee, or
            exercising any trust or power conferred upon the Preferred Guarantee
            Trustee under this Preferred Securities Guarantee; and

                  (iv) no provision of this Preferred Securities Guarantee shall
            require the Preferred Guarantee Trustee to expend or risk its own
            funds or otherwise incur personal financial liability in the
            performance of any of its duties or in the

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            exercise of any of its rights or powers, if the Preferred Guarantee
            Trustee shall have reasonable grounds for believing that the
            repayment of such funds or liability is not reasonably assured to it
            under the terms of this Preferred Securities Guarantee or indemnity,
            reasonably satisfactory to the Preferred Guarantee Trustee, against
            such risk or liability is not reasonably assured to it.

      Section 3.2.Certain Rights of Preferred Guarantee Trustee. Subject to the
provisions of Section 3.1:

            (a) The Preferred Guarantee Trustee may conclusively rely, and shall
      be fully protected in acting or refraining from acting upon, any
      resolution, certificate, statement, instrument, opinion, report, notice,
      request, direction, consent, order, bond, debenture, note, other evidence
      of indebtedness or other paper or document believed by it to be genuine
      and to have been signed, sent or presented by the proper party or parties.

            (b) Any direction or act of the Guarantor contemplated by this
      Preferred Securities Guarantee shall be sufficiently evidenced by a
      Direction (as defined in the Trust Agreement) or an Officers' Certificate.

            (c) Whenever, in the administration of this Preferred Securities
      Guarantee, the Preferred Guarantee Trustee shall deem it desirable that a
      matter be proved or established before taking, suffering or omitting any
      action hereunder, the Preferred Guarantee Trustee (unless other evidence
      is herein specifically prescribed) may, in the absence of bad faith on its
      part, request and conclusively rely upon an Officers' Certificate which,
      upon receipt of such request, shall be promptly delivered by the
      Guarantor.

            (d) The Preferred Guarantee Trustee shall have no duty to see to any
      recording, filing or registration of any instrument (or any rerecording,
      refiling or registration thereof).

            (e) The Preferred Guarantee Trustee may consult with counsel, and
      the written advice or opinion of such counsel with respect to legal
      matters shall be full and complete authorization and protection in respect
      of any action taken, suffered or omitted by it hereunder in good faith and
      in accordance with such advice or opinion. Such counsel may be counsel to
      the Guarantor or any of its Affiliates and may include any of its
      employees. The Preferred Guarantee Trustee shall have the right at any
      time to seek instructions concerning the administration of this Preferred
      Securities Guarantee from any court of competent jurisdiction.

            (f) The Preferred Guarantee Trustee shall be under no obligation to
      exercise any of the rights or powers vested in it by this Preferred
      Securities Guarantee at the request or direction of any Holder, unless
      such Holder shall have provided to the Preferred Guarantee Trustee such
      security and indemnity, reasonably satisfactory to the Preferred Guarantee
      Trustee, against the costs, expenses (including attorneys' fees and
      expenses and the expenses of the Preferred Guarantee Trustee's agents,
      nominees or custodians) and liabilities that might be incurred by it in
      complying with such request or direction, including such reasonable
      advances as may be requested by the Preferred

                                       8
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      Guarantee Trustee; provided that, nothing contained in this Section 3.2(f)
      shall be taken to relieve the Preferred Guarantee Trustee, upon the
      occurrence of an Event of Default, of its obligation to exercise the
      rights and powers vested in it by this Preferred Securities Guarantee.

            (g) The Preferred Guarantee Trustee shall not be bound to make any
      investigation into the facts or matters stated in any resolution,
      certificate, statement, instrument, opinion, report, notice, request,
      direction, consent, order, bond, debenture, note, other evidence of
      indebtedness or other paper or document, but the Preferred Guarantee
      Trustee, in its discretion, may make such further inquiry or investigation
      into such facts or matters as it may see fit, but shall incur no liability
      of any kind by reason of such inquiry or investigation.

            (h) The Preferred Guarantee Trustee may execute any of the trusts or
      powers hereunder or perform any duties hereunder either directly or by or
      through agents, nominees, custodians or attorneys, and the Preferred
      Guarantee Trustee shall not be responsible for any misconduct or
      negligence on the part of any agent or attorney appointed with due care by
      it hereunder.

            (i) Any action taken by the Preferred Guarantee Trustee or its
      agents hereunder shall bind the Holders of the Preferred Securities, and
      the signature of the Preferred Guarantee Trustee or its agents alone shall
      be sufficient and effective to perform any such action. No third party
      shall be required to inquire as to the authority of the Preferred
      Guarantee Trustee to so act or as to its compliance with any of the terms
      and provisions of this Preferred Securities Guarantee, both of which shall
      be conclusively evidenced by the Preferred Guarantee Trustee's or its
      agent's taking such action.

            (j) Whenever in the administration of this Preferred Securities
      Guarantee the Preferred Guarantee Trustee shall deem it desirable to
      receive instructions with respect to enforcing any remedy or right or
      taking any other action hereunder, the Preferred Guarantee Trustee (i) may
      request instructions from the Holders of a Majority in liquidation amount
      of the Preferred Securities, (ii) may refrain from enforcing such remedy
      or right or taking such other action until such instructions are received,
      and (iii) shall be protected in conclusively relying on or acting in
      accordance with such instructions.

            (k) No provision of this Preferred Securities Guarantee shall be
      deemed to impose any duty or obligation on the Preferred Guarantee Trustee
      to perform any act or acts or exercise any right, power, duty or
      obligation conferred or imposed on it in any jurisdiction in which it
      shall be illegal, or in which the Preferred Guarantee Trustee shall be
      unqualified or incompetent in accordance with applicable law, to perform
      any such act or acts or to exercise any such right, power, duty or
      obligation. No permissive power or authority available to the Preferred
      Guarantee Trustee shall be construed to be a duty.

      Section 3.3. Not Responsible for Recitals or Issuance of Guarantee. The
recitals contained in this Guarantee shall be taken as the statements of the
Guarantor, and the Preferred Guarantee Trustee does not assume any
responsibility for their correctness. The Preferred

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Guarantee Trustee makes no representation as to the validity or sufficiency of
this Preferred Securities Guarantee.

                                  ARTICLE IV.

                           PREFERRED GUARANTEE TRUSTEE

      Section 4.1. Preferred Guarantee Trustee; Eligibility.

            (a) There shall at all times be a Preferred Guarantee Trustee which
      shall:

                  (i) not be an Affiliate of the Guarantor; and

                  (ii) be a corporation organized and doing business under the
            laws of the United States of America or any State or Territory
            thereof or of the District of Columbia, or a corporation or Person
            permitted by the Securities and Exchange Commission to act as an
            institutional trustee under the Trust Indenture Act, authorized
            under such laws to exercise corporate trust powers, having a
            combined capital and surplus of at least 50 million U.S. dollars
            ($50,000,000), and subject to supervision or examination by Federal,
            State, Territorial or District of Columbia authority. If such
            corporation publishes reports of condition at least annually,
            pursuant to law or to the requirements of the supervising or
            examining authority referred to above, then, for the purposes of
            this Section 4.1(a)(ii), the combined capital and surplus of such
            corporation shall be deemed to be its combined capital and surplus
            as set forth in its most recent report of condition so published.

            (b) If at any time the Preferred Guarantee Trustee shall cease to be
      eligible to so act under Section 4.1(a), the Preferred Guarantee Trustee
      shall immediately resign in the manner and with the effect set out in
      Section 4.2 (c).

            (c) If the Preferred Guarantee Trustee has or shall acquire any
      "conflicting interest" within the meaning of Section 310(b) of the Trust
      Indenture Act, the Preferred Guarantee Trustee and Guarantor shall in all
      respects comply with the provisions of Section 310(b) of the Trust
      Indenture Act.

      Section 4.2. Appointment, Removal and Resignation of Preferred Guarantee
Trustees.

            (a) Subject to Section 4.2(b), the Preferred Guarantee Trustee may
      be appointed or removed without cause at any time by the Guarantor.

            (b) The Preferred Guarantee Trustee shall not be removed in
      accordance with Section 4.2(a) until a Successor Preferred Guarantee
      Trustee has been appointed and has accepted such appointment by written
      instrument executed by such Successor Preferred Guarantee Trustee and
      delivered to the Guarantor.

            (c) The Preferred Guarantee Trustee appointed to office shall hold
      office until a Successor Preferred Guarantee Trustee shall have been
      appointed or until its removal or resignation. The Preferred Guarantee
      Trustee may resign from office (without need for

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<PAGE>


      prior or subsequent accounting) by an instrument in writing executed by
      the Preferred Guarantee Trustee and delivered to the Guarantor, which
      resignation shall not take effect until a Successor Preferred Guarantee
      Trustee has been appointed and has accepted such appointment by instrument
      in writing executed by such Successor Preferred Guarantee Trustee and
      delivered to the Guarantor and the Preferred Guarantee Trustee.

            (d) If no Successor Preferred Guarantee Trustee shall have been
      appointed and accepted appointment as provided in this Section 4.2 within
      60 days after delivery to the Guarantor of an instrument of resignation or
      removal, the Preferred Guarantee Trustee resigning or being removed may
      petition any court of competent jurisdiction for appointment of a
      Successor Preferred Guarantee Trustee. Such court may thereupon, after
      prescribing such notice, if any, as it may deem proper, appoint a
      Successor Preferred Guarantee Trustee.

            (e) No Preferred Guarantee Trustee shall be liable for the acts or
      omissions to act of any Successor Preferred Guarantee Trustee.

            (f) Upon termination of this Preferred Securities Guarantee or
      removal or resignation of the Preferred Guarantee Trustee pursuant to this
      Section 4.2, the Guarantor shall pay to the Preferred Guarantee Trustee
      all amounts accrued to the date of such termination, removal or
      resignation.

                                   ARTICLE V.

                                    GUARANTEE

      Section 5.1. Guarantee. The Guarantor irrevocably and unconditionally
agrees to pay in full to the Holders the Guarantee Payments (without duplication
of amounts theretofore paid by the Issuer), as and when due, regardless of any
defense, right of set-off or counterclaim that the Issuer may have or assert.
The Guarantor's obligation to make a Guarantee Payment may be satisfied by
direct payment of the required amounts by the Guarantor to the Holders or by
causing the Issuer to pay such amounts to the Holders.

      Section 5.2. Waiver of Notice and Demand. The Guarantor hereby waives
notice of acceptance of this Preferred Securities Guarantee and of any liability
to which it applies or may apply, presentment, demand for payment, any right to
require a proceeding first against the Issuer or any other Person before
proceeding against the Guarantor, protest, notice of nonpayment, notice of
dishonor, notice of redemption and all other notices and demands.

      Section 5.3. Obligations Not Affected. The obligations, covenants,
agreements and duties of the Guarantor under this Preferred Securities Guarantee
shall in no way be affected or impaired by reason of the happening from time to
time of any of the following:

            (a) the release or waiver, by operation of law or otherwise, of the
      performance or observance by the Issuer of any express or implied
      agreement, covenant, term or condition relating to the Preferred
      Securities to be performed or observed by the Issuer;

                                       11
<PAGE>


            (b) the extension of time for the payment by the Issuer of all or
      any portion of the Distributions, Redemption Price, Liquidation
      Distribution or any other sums payable under the terms of the Preferred
      Securities or the extension of time for the performance of any other
      obligation under, arising out of, or in connection with, the Preferred
      Securities (other than an extension of time for payment of Distributions,
      Redemption Price, Liquidation Distribution or other sum payable that
      results from the extension of any interest payment period on the
      Debentures or any extension of the maturity date of the Debentures
      permitted by the Indenture);

            (c) any failure, omission, delay or lack of diligence on the part of
      the Holders to enforce, assert or exercise any right, privilege, power or
      remedy conferred on the Holders pursuant to the terms of the Preferred
      Securities, or any action on the part of the Issuer granting indulgence or
      extension of any kind;

            (d) the voluntary or involuntary liquidation, dissolution, sale of
      any collateral, receivership, insolvency, bankruptcy, assignment for the
      benefit of creditors, reorganization, arrangement, composition or
      readjustment of debt of, or other similar proceedings affecting, the
      Issuer or any of the assets of the Issuer;

            (e) any invalidity of, or defect or deficiency in, the Preferred
      Securities;

            (f) the settlement or compromise of any obligation guaranteed hereby
      or hereby incurred; or

            (g) any other circumstance whatsoever that might otherwise
      constitute a legal or equitable discharge or defense of a guarantor, it
      being the intent of this Section 5.3 that the obligations of the Guarantor
      hereunder shall be absolute and unconditional under any and all
      circumstances.

      There shall be no obligation of the Holders to give notice to, or obtain
consent of, the Guarantor with respect to the happening of any of the foregoing.

      Section 5.4. Rights of Holders.

            (a) The Holders of a Majority in liquidation amount of the Preferred
      Securities have the right to direct the time, method and place of
      conducting of any proceeding for any remedy available to the Preferred
      Guarantee Trustee in respect of this Preferred Securities Guarantee or
      exercising any trust or power conferred upon the Preferred Guarantee
      Trustee under this Preferred Securities Guarantee.

            (b) If the Preferred Guarantee Trustee fails to enforce this
      Preferred Securities Guarantee with respect to the right of any Holders to
      receive any Guarantee Payment in accordance herewith, such Holders may
      directly institute a proceeding against the Guarantor for enforcement of
      the Preferred Security Guarantee for such payment. The Guarantor waives
      any right or remedy to require that any action be brought first against
      the Issuer or any other Person or entity before proceeding directly
      against the Guarantor.

                                       12
<PAGE>


      Section 5.5. Guarantee of Payment. This Preferred Securities Guarantee
creates a guarantee of payment and not of collection.

      Section 5.6. Subrogation. The Guarantor shall be subrogated to all (if
any) rights of the Holders of Preferred Securities against the Issuer in respect
of any amounts paid to such Holders by the Guarantor under this Preferred
Securities Guarantee; provided, however, that the Guarantor shall not (except to
the extent required by mandatory provisions of law) be entitled to enforce or
exercise any right that it may acquire by way of subrogation or any indemnity,
reimbursement or other agreement, in all cases as a result of payment under this
Preferred Securities Guarantee, if, at the time of any such payment, any amounts
are due and unpaid under this Preferred Securities Guarantee. If any amount
shall be paid to the Guarantor in violation of the preceding sentence, the
Guarantor agrees to hold such amount in trust for the Holders and to pay over
such amount to the Holders.

      Section 5.7. Independent Obligations. The Guarantor acknowledges that its
obligations hereunder are independent of the obligations of the Issuer with
respect to the Preferred Securities, and that the Guarantor shall be liable as
principal and as debtor hereunder to make Guarantee Payments pursuant to the
terms of this Preferred Securities Guarantee notwithstanding the occurrence of
any event referred to in subsections (a) through (g), inclusive, of Section 5.3
hereof.

      Section 5.8. Taxes. All payments in respect of the Guarantee Payments
(including interest accrued thereon, if any) by the Guarantor shall be made
without withholding or deduction for or on account of any Relevant Tax, unless
the withholding or deduction of such Relevant Tax is required by law. In that
event, the Guarantor shall pay, as further Guarantee Payments, such additional
amounts as may be necessary in order that the net amounts received by a Holder
(or a third party on its behalf) after such withholding or deduction will equal
the amount which would have been received in respect of the Guarantee Payments
(including interest accrued thereon, if any) in the absence of such withholding
or deduction ("Guarantee Additional Amounts"), except that no such Guarantee
Additional Amounts shall be payable to a Holder (or a third party on its behalf)
with respect to any Guarantee Payments (including interest accrued thereon, if
any) (i) to the extent that such Relevant Tax is imposed or levied by virtue of
such Holder (or the beneficial owner of Common Securities to which such
Guarantee Payments relate) having some connection with the Relevant
Jurisdiction, other than being a Holder (or beneficial owner) of Common
Securities ) or (ii) to the extent that such Relevant Tax is imposed or levied
by virtue of such Holder (or beneficial owner) not having made a declaration of
non-residence in, or other lack of connection with, the Relevant Jurisdiction of
any similar claim for exemption, if the Guarantor or its agent has provided the
beneficial owner of such Common Securities or its nominee with at least 60 days'
prior written notice of any opportunity to make such a declaration or claim.

                                  ARTICLE VI.

                    LIMITATION OF TRANSACTIONS; SUBORDINATION

      Section 6.1. Limitation of Transactions. So long as any Preferred
Securities remain outstanding, if there shall have occurred and be continuing an
Event of Default or an event of

                                       13
<PAGE>


default under the Trust Agreement, then (a) the Guarantor shall not declare or
pay any dividend on, make any distributions with respect to, or redeem,
purchase, acquire or make a liquidation payment with respect to, any of its
capital stock (other than (i) purchases or acquisitions of shares of its common
stock in connection with the satisfaction by the Guarantor of its obligations
under any employee benefit plans or the satisfaction by the Guarantor of its
obligations pursuant to any contract or security requiring the Guarantor to
purchase shares of its common stock, (ii) as a result of a reclassification of
its capital stock or the exchange or conversion of one class or series of its
capital stock for another class or series of its capital stock, (iii) the
purchase of fractional interests in shares of its capital stock pursuant to the
conversion or exchange provisions of such capital stock or the security being
converted or exchanged, and (iv) redemptions or purchases pursuant to the
Guarantor's Rights Agreement, dated as of May 11, 1999, as amended, between the
Guarantor and Wells Fargo Bank Minnesota, N.A., as successor in interest to
ChaseMellon Shareholder Services, LLC), (b) the Guarantor shall not make any
payment of interest, principal or premium, if any, on or repay, repurchase or
redeem any debt securities (including guarantees) issued by the Guarantor which
rank pari passu with or junior to the Debentures and (c) the Guarantor shall not
make any guarantee payments with respect to the foregoing (other than pursuant
to this Agreement and the Common Securities Guarantee, dated as of July 26, 2005
of the Guarantor with respect to the 6.750% Trust Originated Common Securities
of the Trust), the Common Securities Guarantee Agreement, dated as of December
20, 1996 and the Capital Securities Guarantee Agreement, dated as of June 6,
1997 (the "Capital Guarantee")).

      Section 6.2. Subordinated Ranking. This Preferred Securities Guarantee
will constitute an unsecured obligation of the Guarantor and will rank (i)
subordinate and junior in right of payment to all Senior Indebtedness (as such
term is defined in the Indenture), (ii) pari passu with the Debentures, the
8.197% Junior Subordinated Deferrable Interest Debentures due May, 2045, the
Guarantor's Capital Guarantee and any other liabilities or obligations that may
be pari passu by their terms and (iii) senior to the Guarantor's common stock,
the Common Securities Guarantee, the Common Securities Guarantee Agreement dated
as of December 20, 1996, the most senior preferred or preference stock now or
hereafter issued by the Guarantor and with any guarantee now or hereafter
entered into by the Guarantor in respect of any preferred or preference stock of
any affiliate of the Guarantor.

                                  ARTICLE VII.

                                   TERMINATION

      Section 7.1. Termination. This Preferred Securities Guarantee shall
terminate upon (i) full payment of the Redemption Price of all Preferred
Securities, (ii) upon the distribution of the Debentures to the Holders of all
of the Preferred Securities or (iii) upon full payment of the amounts payable in
accordance with the Trust Agreement upon liquidation of the Issuer.
Notwithstanding the foregoing, this Preferred Securities Guarantee will continue
to be effective or will be reinstated, as the case may be, if at any time any
Holder of Preferred Securities must restore payment of any sums paid under the
Preferred Securities or under this Preferred Securities Guarantee.

                                       14
<PAGE>


                                 ARTICLE VIII.

                                INDEMNIFICATION

      Section 8.1. Exculpation. (a) No Indemnified Person shall be liable,
responsible or accountable in damages or otherwise to the Guarantor or any
Covered Person for any loss, damage or claim incurred by reason of any act or
omission performed or omitted by such Indemnified Person in good faith in
accordance with this Preferred Securities Guarantee and in a manner that such
Indemnified Person reasonably believed to be within the scope of the authority
conferred on such Indemnified Person by this Preferred Securities Guarantee or
by law, except that an Indemnified Person shall be liable for any such loss,
damage or claim incurred by reason of such Indemnified Person's negligence or
willful misconduct with respect to such acts or omissions.

            (b) An Indemnified Person shall be fully protected in relying in
      good faith upon the records of the Guarantor and upon such information,
      opinions, reports or statements presented to the Guarantor by any Person
      as to matters the Indemnified Person reasonably believes are within such
      other Person's professional or expert competence and who has been selected
      with reasonable care by or on behalf of the Guarantor, including
      information, opinions, reports or statements as to the value and amount of
      the assets, liabilities, profits, losses, or any other facts pertinent to
      the existence and amount of assets from which Distributions to Holders of
      Preferred Securities might properly be paid.

      Section 8.2. Indemnification.

            (a) To the fullest extent permitted by applicable law, the Guarantor
      shall indemnify and hold harmless each Indemnified Person from and against
      any loss, damage or claim incurred by such Indemnified Person by reason of
      any act or omission performed or omitted by such Indemnified Person in
      good faith in accordance with this Guarantee Agreement and in a manner
      such Indemnified Person reasonably believed to be within the scope of
      authority conferred on such Indemnified Person by this Guarantee
      Agreement, except that no Indemnified Person shall be entitled to be
      indemnified in respect of any loss, damage or claim incurred by such
      Indemnified Person by reason of negligence or willful misconduct with
      respect to such acts or omissions.

            (b) To the fullest extent permitted by applicable law, reasonable
      expenses (including legal fees) incurred by an Indemnified Person in
      defending any claim, demand, action, suit or proceeding shall, from time
      to time, be advanced by the Guarantor prior to the final disposition of
      such claim, demand, action, suit or proceeding upon receipt by the
      Guarantor of an undertaking by or on behalf of the Indemnified Person to
      repay such amount if it shall be determined that the Indemnified Person is
      not entitled to be indemnified as authorized in Section 8.2(a).

            (c) The obligation to indemnify as set forth in this Section 8.2
      shall survive the termination of the Preferred Securities Guarantee.

                                       15
<PAGE>


                                  ARTICLE IX.

                                 MISCELLANEOUS

      Section 9.1. Successors and Assigns. All guarantees and agreements
contained in this Preferred Securities Guarantee shall bind the successors,
assigns, receivers, trustees and representatives of the Guarantor and shall
inure to the benefit of the Holders of the Preferred Securities then
outstanding.

      Section 9.2. Amendments. Except with respect to any changes that do not
adversely affect the rights of Holders (in which case no consent of Holders will
be required), this Preferred Securities Guarantee may only be amended with the
prior approval of the Holders of at least a Majority in liquidation amount
(including the stated amount that would be paid on redemption, liquidation or
otherwise, plus accrued and unpaid Distributions to the date upon which the
voting percentages are determined) of all the outstanding Preferred Securities.
The provisions of Section 12.2 of the Trust Agreement with respect to meetings
of Holders of the Securities apply to the giving of such approval.

      Section 9.3. Notices. All notices provided for in this Preferred
Securities Guarantee shall be in writing, duly signed by the party giving such
notice, and shall be delivered, telecopied or mailed by registered or certified
mail, as follows:

            (a) if given to the Preferred Guarantee Trustee, at the Preferred
      Guarantee Trustee's mailing address set forth below (or such other address
      as the Preferred Guarantee Trustee may give notice of to the Holders of
      the Preferred Securities):

                The Bank of New York
                101 Barclay Street, 8W
                New York, New York 10286
                Facsimile: (212) 815-5704
                Attention: Geovanni Barris

            (b) If given to the Guarantor, at the Guarantor's mailing address
      set forth below (or such other address as the Guarantor may give notice of
      to the Holders of the Preferred Securities):

                W. R. Berkley Corporation
                475 Steamboat Road
                Greenwich, Connecticut 06830
                Facsimile No.: (203) 769-4098
                Attention: Ira S. Lederman

            (c) If given to any Holder of Preferred Securities, at the address
      set forth on the books and records of the Issuer.

      All such notices shall be deemed to have been given when received in
person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid except that if a notice or other document is refused delivery or
cannot be delivered because of a changed address of which

                                       16
<PAGE>


no notice was given, such notice or other document shall be deemed to have been
delivered on the date of such refusal or inability to deliver.

      Section 9.4. Benefit. This Preferred Securities Guarantee is solely for
the benefit of the Holders of the Preferred Securities and, subject to Section
3.1(a), is not separately transferable from the Preferred Securities.

      Section 9.5. Governing Law. THIS PREFERRED SECURITIES GUARANTEE SHALL BE
GOVERNED BY, AND CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE
STATE OF NEW YORK.

                                       17
<PAGE>


      THIS PREFERRED SECURITIES GUARANTEE is executed as of the day and year
first above written.

                                        W. R. BERKLEY CORPORATION, as Guarantor

                                        By: /s/ Eugene G. Ballard
                                            ------------------------------------
                                           Name: Eugene G. Ballard
                                           Title: Senior Vice President

                                        THE BANK OF NEW YORK, as Preferred
                                        Guarantee Trustee

                                        By: /s/ Stacey B. Poindexter
                                            ------------------------------------
                                           Name: Stacey B. Poindexter
                                           Title: Assistant Vice President

                                       18