Document:

<PAGE>   1
                                                                    Exhibit 4.16

                                 TRUST AGREEMENT

                                       OF

                        TRENWICK AMERICA CAPITAL TRUST II

        THIS TRUST AGREEMENT is made as of June 15, 2001 (this "Trust
Agreement"), by and among Trenwick America Corporation, a Delaware corporation,
as Depositor (the "Depositor"), and Bank One Trust Company, N.A., a national
banking association , as property trustee (the "Property Trustee"), Bank One
Delaware, Inc., a Delaware corporation, as trustee (the "Delaware Trustee"),
John V. Del Col, an individual, as trustee, Yvonne M. Poster, an individual, as
trustee, and David M. Finkelstein, an individual, as trustee (jointly, the
"Administrative Trustees") (the Property Trustee, the Delaware Trustee and the
Administrative Trustees are hereinafter collectively referred to as the
"Trustees"). The Depositor and the Trustees hereby agree as follows:

        1. The trust created hereby shall be known as "Trenwick America Capital
Trust II" (the "Trust"), in which name the Trustees or the Depositor, to the
extent provided herein, may conduct the business of the Trust, make and execute
contracts, and sue and be sued.

        2. The Depositor hereby assigns, transfers, conveys and sets over to the
Trust the sum of $10. Such amount shall constitute the initial trust estate. It
is the intention of the parties hereto that the Trust created hereby constitute
a business trust under Chapter 38 of Title 12 of the Delaware Code, 12 Del. C.
Section 3801, et seq. (the "Business Trust Act"), and that this document
constitute the governing instrument of the Trust. The Trustees are hereby
authorized and directed to execute and file a certificate of trust with the
Delaware Secretary of State in such form as the Trustees may approve.

        3. The Depositor and the Trustees expect to enter into an amended and
restated trust agreement satisfactory to each such party to provide for the
contemplated operation of the Trust created hereby and the issuance of the
Preferred Securities and Common Securities referred to therein. Prior to the
execution and delivery of such amended and restated trust agreement, the
Trustees shall not have any duty or obligation hereunder or with respect of the
trust estate, except as otherwise required by applicable law or as may be
necessary to obtain prior to such execution and delivery any licenses, consents
or approvals required by applicable law or otherwise. Notwithstanding the
foregoing, the Trustees may take all actions deemed proper as are necessary to
effect the transactions contemplated herein.

        4. The Depositor, as depositor of the Trust, is hereby authorized, in
its discretion, (i) to prepare and file with the Securities and Exchange
Commission (the "Commission") and to execute, in the case of the 1933 Act
Registration Statement and 1934 Act Registration Statement (each as herein
defined), on behalf of the Trust, (a) a Registration Statement (the "1933 Act
Registration Statement"), including all pre-effective and post-effective
amendments thereto,

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<PAGE>   2

relating to the registration under the Securities Act of 1933, as amended (the
"1933 Act"), of the Preferred Securities of the Trust, (b) any preliminary
prospectus or prospectus or supplement thereto relating to the Preferred
Securities of the Trust required to be filed pursuant to the 1933 Act, and (c) a
Registration Statement on Form 8-A or other appropriate form (the "1934 Act
Registration Statement"), including all pre-effective and post-effective
amendments thereto, relating to the registration of the Preferred Securities of
the Trust under the Securities Exchange Act of 1934, as amended; (ii) if and at
such time as determined by the Depositor, to file with the New York Stock
Exchange or other exchange, or the National Association of Securities Dealers
("NASD"), and execute on behalf of the Trust a listing application and all other
applications, statements, certificates, agreements and other instruments as
shall be necessary or desirable to cause the Preferred Securities of the Trust
to be listed on the New York Stock Exchange or such other exchange, or the
NASD's Nasdaq National Market; (iii) to file and execute on behalf of the Trust,
such applications, reports, surety bonds, irrevocable consents, appointments of
attorney for service of process and other papers and documents that shall be
necessary or desirable to register the Preferred Securities of the Trust under
the securities or "Blue Sky" laws of such jurisdictions as the Depositor, on
behalf of the Trust, may deem necessary or desirable; (iv) to execute and
deliver letters or documents to, or instruments for filing with, a depository
relating to the Preferred Securities of the Trust; (v) to execute, deliver and
perform on behalf of the Trust an underwriting agreement with one or more
underwriters relating to the offering of the Preferred Securities of the Trust;
and (vi) to execute on behalf of the Trust such purchase agreements with one or
more purchasers or agents relating to an offering of securities of the Trust as
the Depositor, in its sole discretion on behalf of the Trust, may deem necessary
or desireable.

        In the event that any filing referred to in this Section 4 is required
by the rules and regulations of the Commission, the New York Stock Exchange or
other exchange, NASD, or state securities or "Blue Sky" laws to be executed on
behalf of the Trust by the Trustees, the Administrative Trustees, in their
capacity as trustees of the Trust, are hereby authorized to join in any such
filing and to execute on behalf of the Trust any and all of the foregoing.

        5. This Trust Agreement may be executed in one or more counterparts.

        6. The number of trustees of the Trust initially shall be five (5) and
thereafter the number of trustees of the Trust shall be such number as shall be
fixed from time to time by a written instrument signed by the Depositor which
may increase or decrease the number of trustees of the Trust; provided, however,
that to the extent required by the Business Trust Act, one trustee of the Trust
shall either be a natural person who is a resident of the State of Delaware or,
if not a natural person, an entity which has its principal place of business in
the State of Delaware and otherwise meets the requirements of applicable
Delaware law. Subject to the foregoing, the Depositor is entitled to appoint or
remove without cause any trustee of the Trust at any time. Any trustee of the
Trust may resign upon thirty days' prior notice to the Depositor.

        7. Bank One Delaware, Inc., in its capacity as Trustee, shall not have
the powers or duties of the Trustees set forth herein (except as may be required
under the Business Trust Act) and shall be a trustee hereunder for the sole and
limited purpose of fulfilling the requirements of Section 3807(a) of the
Business Trust Act.

        8. The Trust may be dissolved and terminated before the issuance of the
Preferred

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<PAGE>   3

Securities at the election of the Depositor.

        9. This Trust Agreement shall be governed by, and construed in
accordance with, the laws of the State of Delaware (without regard to conflict
of laws principles).

        10. The Depositor hereby agrees to (i) reimburse the Trustees for all
reasonable expenses (including reasonable fees and expenses of counsel and other
experts) and (ii) indemnify, defend and hold harmless the Trustees and any of
the officers, directors, employees and agents of the Trustees (the 'Indemnified
Persons") from and against any losses, damages, liabilities, claims, actions,
suits, costs, expenses, disbursements (including the reasonable fees and
expenses of counsel), taxes and penalties of any kind and nature whatsoever
(collectively, "Expenses"), to the extent that such Expenses arise out of or are
imposed upon or asserted at any time against such Indemnified Persons with
respect to the performance of this Trust Agreement, the creation, operation or
termination of the Trust or the transactions contemplated hereby; provided,
however, that the Depositor shall not be required to indemnify any Indemnified
Person for any Expenses which are a result of the willful misconduct, bad faith
or gross negligence of such Indemnified Person.

                                       3
<PAGE>   4

        IN WITNESS WHEREOF, the parties hereto have caused this Trust Agreement
to be duly executed as of the day and year first above written.

                           TRENWICK AMERICA CORPORATION, as
                           Depositor

                           By:           /s/ Alan L. Hunte
                               ---------------------------------------
                               Name:     /s/ Alan L. Hunte
                                       -------------------------------
                               Title:    Chief Accounting Officer
                                       -------------------------------

                           BANK ONE TRUST COMPANY, N.A., as trustee
                              of the Trust

                           By:         /s/ Sandra L. Caruba
                               ---------------------------------------
                               Name:     Sandra L. Caruba
                                       -------------------------------
                               Title:    Vice President
                                       -------------------------------

                           BANK ONE DELAWARE, INC., as trustee of the
                           Trust

                           By:         /s/ Sandra L. Caruba
                               ---------------------------------------
                               Name:     Sandra L. Caruba
                                       -------------------------------
                               Title:    Vice President
                                       -------------------------------

                           JOHN V. DEL COL, as Administrative Trustee

                                       /s/ John V. Del Col
                           -------------------------------------------
                                        John V. Del Col

                           YVONNE M. POSTER, as Administrative Trustee

                                       /s/ Yvonne M. Poster
                           -------------------------------------------
                                        Yvonne M. Poster

                           DAVID M. FINKELSTEIN, as Administrative
                             Trustee

                                       /s/ David M. Finkelstein
                           -------------------------------------------
                                        David M. Finkelstein

                                       4<PAGE>   1
                                                                    Exhibit 4.17

                                 TRUST AGREEMENT

                                       OF

                       TRENWICK AMERICA CAPITAL TRUST III

        THIS TRUST AGREEMENT is made as of June 15, 2001 (this "Trust
Agreement"), by and among Trenwick America Corporation, a Delaware corporation,
as Depositor (the "Depositor"), and Bank One Trust Company, N.A., a national
banking association , as property trustee (the "Property Trustee"), Bank One
Delaware, Inc., a Delaware corporation, as trustee (the "Delaware Trustee"),
John V. Del Col, an individual, as trustee, Yvonne M. Poster, an individual, as
trustee, and David M. Finkelstein, an individual, as trustee (jointly, the
"Administrative Trustees") (the Property Trustee, the Delaware Trustee and the
Administrative Trustees are hereinafter collectively referred to as the
"Trustees"). The Depositor and the Trustees hereby agree as follows:

        1. The trust created hereby shall be known as "Trenwick America Capital
Trust III" (the "Trust"), in which name the Trustees or the Depositor, to the
extent provided herein, may conduct the business of the Trust, make and execute
contracts, and sue and be sued.

        2. The Depositor hereby assigns, transfers, conveys and sets over to the
Trust the sum of $10. Such amount shall constitute the initial trust estate. It
is the intention of the parties hereto that the Trust created hereby constitute
a business trust under Chapter 38 of Title 12 of the Delaware Code, 12 Del. C.
Section 3801, et seq. (the "Business Trust Act"), and that this document
constitute the governing instrument of the Trust. The Trustees are hereby
authorized and directed to execute and file a certificate of trust with the
Delaware Secretary of State in such form as the Trustees may approve.

        3. The Depositor and the Trustees expect to enter into an amended and
restated trust agreement satisfactory to each such party to provide for the
contemplated operation of the Trust created hereby and the issuance of the
Preferred Securities and Common Securities referred to therein. Prior to the
execution and delivery of such amended and restated trust agreement, the
Trustees shall not have any duty or obligation hereunder or with respect of the
trust estate, except as otherwise required by applicable law or as may be
necessary to obtain prior to such execution and delivery any licenses, consents
or approvals required by applicable law or otherwise. Notwithstanding the
foregoing, the Trustees may take all actions deemed proper as are necessary to
effect the transactions contemplated herein.

        4. The Depositor, as depositor of the Trust, is hereby authorized, in
its discretion, (i) to prepare and file with the Securities and Exchange
Commission (the "Commission") and to execute, in the case of the 1933 Act
Registration Statement and 1934 Act Registration Statement (each as herein
defined), on behalf of the Trust, (a) a Registration Statement (the "1933 Act
Registration Statement"), including all pre-effective and post-effective
amendments thereto,

                                       1
<PAGE>   2

relating to the registration under the Securities Act of 1933, as amended (the
"1933 Act"), of the Preferred Securities of the Trust, (b) any preliminary
prospectus or prospectus or supplement thereto relating to the Preferred
Securities of the Trust required to be filed pursuant to the 1933 Act, and (c) a
Registration Statement on Form 8-A or other appropriate form (the "1934 Act
Registration Statement"), including all pre-effective and post-effective
amendments thereto, relating to the registration of the Preferred Securities of
the Trust under the Securities Exchange Act of 1934, as amended; (ii) if and at
such time as determined by the Depositor, to file with the New York Stock
Exchange or other exchange, or the National Association of Securities Dealers
("NASD"), and execute on behalf of the Trust a listing application and all other
applications, statements, certificates, agreements and other instruments as
shall be necessary or desirable to cause the Preferred Securities of the Trust
to be listed on the New York Stock Exchange or such other exchange, or the
NASD's Nasdaq National Market; (iii) to file and execute on behalf of the Trust,
such applications, reports, surety bonds, irrevocable consents, appointments of
attorney for service of process and other papers and documents that shall be
necessary or desirable to register the Preferred Securities of the Trust under
the securities or "Blue Sky" laws of such jurisdictions as the Depositor, on
behalf of the Trust, may deem necessary or desirable; (iv) to execute and
deliver letters or documents to, or instruments for filing with, a depository
relating to the Preferred Securities of the Trust; (v) to execute, deliver and
perform on behalf of the Trust an underwriting agreement with one or more
underwriters relating to the offering of the Preferred Securities of the Trust;
and (vi) to execute on behalf of the Trust such purchase agreements with one or
more purchasers or agents relating to an offering of securities of the Trust as
the Depositor, in its sole discretion on behalf of the Trust, may deem necessary
or desireable.

        In the event that any filing referred to in this Section 4 is required
by the rules and regulations of the Commission, the New York Stock Exchange or
other exchange, NASD, or state securities or "Blue Sky" laws to be executed on
behalf of the Trust by the Trustees, the Administrative Trustees, in their
capacity as trustees of the Trust, are hereby authorized to join in any such
filing and to execute on behalf of the Trust any and all of the foregoing.

        5. This Trust Agreement may be executed in one or more counterparts.

        6. The number of trustees of the Trust initially shall be five (5) and
thereafter the number of trustees of the Trust shall be such number as shall be
fixed from time to time by a written instrument signed by the Depositor which
may increase or decrease the number of trustees of the Trust; provided, however,
that to the extent required by the Business Trust Act, one trustee of the Trust
shall either be a natural person who is a resident of the State of Delaware or,
if not a natural person, an entity which has its principal place of business in
the State of Delaware and otherwise meets the requirements of applicable
Delaware law. Subject to the foregoing, the Depositor is entitled to appoint or
remove without cause any trustee of the Trust at any time. Any trustee of the
Trust may resign upon thirty days' prior notice to the Depositor.

        7. Bank One Delaware, Inc., in its capacity as Trustee, shall not have
the powers or duties of the Trustees set forth herein (except as may be required
under the Business Trust Act) and shall be a trustee hereunder for the sole and
limited purpose of fulfilling the requirements of Section 3807(a) of the
Business Trust Act.

        8. The Trust may be dissolved and terminated before the issuance of the
Preferred

                                       2
<PAGE>   3

Securities at the election of the Depositor.

        9. This Trust Agreement shall be governed by, and construed in
accordance with, the laws of the State of Delaware (without regard to conflict
of laws principles).

        10. The Depositor hereby agrees to (i) reimburse the Trustees for all
reasonable expenses (including reasonable fees and expenses of counsel and other
experts) and (ii) indemnify, defend and hold harmless the Trustees and any of
the officers, directors, employees and agents of the Trustees (the 'Indemnified
Persons") from and against any losses, damages, liabilities, claims, actions,
suits, costs, expenses, disbursements (including the reasonable fees and
expenses of counsel), taxes and penalties of any kind and nature whatsoever
(collectively, "Expenses"), to the extent that such Expenses arise out of or are
imposed upon or asserted at any time against such Indemnified Persons with
respect to the performance of this Trust Agreement, the creation, operation or
termination of the Trust or the transactions contemplated hereby; provided,
however, that the Depositor shall not be required to indemnify any Indemnified
Person for any Expenses which are a result of the willful misconduct, bad faith
or gross negligence of such Indemnified Person.

                                       3
<PAGE>   4

        IN WITNESS WHEREOF, the parties hereto have caused this Trust Agreement
to be duly executed as of the day and year first above written.

                             TRENWICK AMERICA CORPORATION, as
                             Depositor

                             By:           /s/ Alan L. Hunte
                                 ---------------------------------------
                                 Name:     /s/ Alan L. Hunte
                                         -------------------------------
                                 Title:    Chief Accounting Officer
                                         -------------------------------

                             BANK ONE TRUST COMPANY, N.A., as trustee
                                of the Trust

                             By:         /s/ Sandra L. Caruba
                                 ---------------------------------------
                                 Name:     Sandra L. Caruba
                                         -------------------------------
                                 Title:    Vice President
                                         -------------------------------

                             BANK ONE DELAWARE, INC., as trustee of the
                             Trust

                             By:         /s/ Sandra L. Caruba
                                 ---------------------------------------
                                 Name:     Sandra L. Caruba
                                         -------------------------------
                                 Title:    Vice President
                                         -------------------------------

                             JOHN V. DEL COL, as Administrative Trustee

                                         /s/ John V. Del Col
                             -------------------------------------------
                                          John V. Del Col

                             YVONNE M. POSTER, as Administrative Trustee

                                         /s/ Yvonne M. Poster
                             -------------------------------------------
                                          Yvonne M. Poster

                             DAVID M. FINKELSTEIN, as Administrative
                               Trustee

                                         /s/ David M. Finkelstein
                             -------------------------------------------
                                          David M. Finkelstein

                                       4

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