<PAGE>

                                                                    EXHIBIT 4.85

                                                          CLIFFORD CHANCE US LLP

                                                                  CONFORMED COPY

                          MARCONI COMMUNICATIONS, INC.
                                   AS PLEDGOR

                                       AND

                   THE LAW DEBENTURE TRUST CORPORATION p.l.c.,
                                AS SECURED PARTY

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                             STOCK PLEDGE AGREEMENT

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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
SECTION                                                                                                 PAGE
<S>                                                                                                     <C>
Section 1.   Definitions...............................................................................    1

Section 2.   Pledge....................................................................................    2

Section 3.   Representations and Warranties of Pledgor.................................................    4

Section 4.   Covenants of Pledgor......................................................................    5

Section 5.   Voting Rights and Certain Payments Prior to Enforcement Event.............................    5

Section 6.   All Payments in Trust.....................................................................    6

Section 7.   Expenses..................................................................................    6

Section 8.   Remedies..................................................................................    7

Section 9.   Suretyship Waivers by Pledgor; Obligations Absolute......................................    10

Section 10.  Marshalling..............................................................................    10

Section 11.  Proceeds of Dispositions.................................................................    11

Section 12.  termination..............................................................................    11

Section 13.  Reinstatement............................................................................    11

Section 14.  Miscellaneous............................................................................    11
</TABLE>

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         STOCK PLEDGE AGREEMENT (this "AGREEMENT") dated as of May 19, 2003,
between MARCONI COMMUNICATIONS, INC., a Delaware corporation ("PLEDGOR"), and
THE LAW DEBENTURE TRUST CORPORATION p.l.c. ("LAW DEBENTURE"), as Security
Trustee (together with any co-trustee, co-agent or other entity appointed
pursuant to clause 16 of the STID (as defined below), "SECURED PARTY") under the
Security Trust and Intercreditor Deed dated the date hereof and made among
Marconi Corporation plc as Issuer; Secured Party; the persons listed in Schedule
1 thereto as Guarantors; Law Debenture Trust Company of New York as Senior Note
Trustee; JPMorgan Chase Bank as Junior Note Trustee; HSBC Bank plc as New
Bonding Facility Agent and Escrow Bank; The Bank of New York as Depositary,
Paying Agent and Registrar; the persons listed in Part A of Schedule 2 thereto
as Intra-Group Creditors; the persons listed in Part B of Schedule 2 thereto as
Intra-Group Borrowers; and the persons listed in Schedule 3 thereto as New
Bonding Facility Banks (as amended, modified or supplemented from time to time,
the "STID"). Capitalized terms used herein without definition shall have the
respective meanings assigned to such terms in the STID.

         WHEREAS, Pledgor has entered into (i) the Guarantee of Senior Notes,
(ii) the Guarantee of Junior Notes and (iii) the Composite Guarantee, each dated
the date hereof in favor of Secured Party (collectively, the "GUARANTEES"),
pursuant to which Pledgor has agreed to guarantee certain obligations and to
secure those obligations and, in connection therewith, inter alia, to execute
and deliver to Secured Party a stock pledge agreement in substantially the form
hereof;

         NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

         Section 1.     DEFINITIONS.

                  1.01.    Definition of Terms Used Herein Generally. Unless
otherwise defined herein, terms defined in the STID and used herein have the
meanings given to them in the STID. All terms used herein and defined in the
NYUCC shall have the same definitions herein as specified therein; provided,
however, that if a term is defined in Article 9 of the NYUCC differently than in
another Article of the NYUCC, the term has the meaning specified in Article 9 of
the NYUCC.

                  1.02.    Definition of Certain Terms Used Herein. As used
herein, the following terms shall have the following meanings:

         "LIEN" shall have the meaning assigned to the term "Security" in the
STID.

         "PLEDGED COLLATERAL" shall have the meaning assigned to such term in
Section 2.01.

         "PLEDGED SECURITIES" shall have the meaning assigned to such term in
Section 2.02(b).

         "NYUCC" shall mean the Uniform Commercial Code as in effect in the
State of New York from time to time.

         "SECURED OBLIGATIONS" shall mean all present and future indebtedness,
liabilities and obligations (for the avoidance of doubt, including any
liabilities and obligations which have been cash-collateralized by the Pledgor)
at any time of the Pledgor under the Relevant Documents, both actual and
contingent and

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                                                                               2

whether incurred solely or jointly or in any other capacity together with any of
the following matters relating to or arising in respect of those liabilities and
obligations:.

         (a)      any refinancing, novation, deferral or extension;

         (b)      any obligation relating to any increase in the amount of such
                  obligations;

         (c)      any claim for damages or restitution; and

         (d)      any claim as a result of any recovery by the Pledgor of a
                  payment or discharge, or non- allowability, on the grounds of
                  preference,

and any amounts that would be included in any of the above but for any
discharge, non-provability or unenforceability of those amounts in any
insolvency or other proceedings (including interest accruing after the
commencement of any insolvency or other proceedings).

         "SECURITIES ACT" shall have the meaning assigned to such term in
Section 8.01(d).

         "SECURITY INTERESTS" means the security interests created under this
Agreement.

         "STID" shall have the meaning assigned to such term in the preamble of
this Agreement.

         "UCC" means the Uniform Commercial Code as in effect in any
jurisdiction. References to particular sections of Article 9 of the UCC shall
be, unless otherwise indicated, references to Revised Article 9 of the UCC
adopted and effective in certain jurisdictions on or after July 1, 2001.

                  1.03.    Rules of Interpretation.

                           (a) The rules of interpretation specified in clause
1.2 of the STID shall be applicable to this Agreement.

                           (b) "continuing" in relation to an Enforcement Event
shall be construed as a reference to an acceleration of any Secured Obligation
(other than Secured Obligations arising under the New Bonding Facility
Agreement) where such acceleration has not been rescinded in writing or a
declaration that the Secured Obligations (other than Secured Obligations arising
under the New Bonding Facility Agreement) are prematurely due and payable (other
than solely as a result of it becoming unlawful for a Secured Creditor to
perform its obligations under the Relevant Documents) where such declaration has
not been revoked in writing or any failure by an Obligor to pay any principal
amount in respect of any Secured Obligations (other than Secured Obligations
arising under the New Bonding Facility Agreement) whether on maturity or
otherwise which has not been waived in writing. For purposes of this clause,
"Secured Obligations" shall have the meaning assigned to such term in the STID.

                  1.04.    STID. The parties hereto acknowledge that Secured
Party, when acting hereunder, shall be acting in accordance with and subject to
the terms of the STID.

                  1.05.    Conflict. Notwithstanding any provision to the
contrary contained herein, the parties agree that this Agreement is subject in
all respects to the STID and for the avoidance of doubt, in the event of any
inconsistency, the provisions of the STID shall prevail.

         Section 2.    PLEDGE.

                             STOCK PLEDGE AGREEMENT

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                                                                               3

                  2.01.    Grant of Security Interest. To secure the payment or
performance, as the case may be, in full of the Secured Obligations, whether at
stated maturity, by acceleration or otherwise, Pledgor hereby pledges to Secured
Party, and grants to Secured Party a first priority Security Interest in, the
collateral described in Section 2.02 (collectively, the "PLEDGED COLLATERAL").

                  2.02.    Description of Pledged Collateral.

                           (a) The Pledged Collateral is described as follows
and on any separate schedules at any time furnished by Pledgor to Secured Party
(which schedules are hereby deemed part of this Agreement):

                                    (i)      all right, title and interest of
Pledgor as a holder (whether now or in the future) in (x) shares or other equity
interests in any corporations, limited liability companies or other entities
(including, without limitation, those entities described on Schedule 1)
organized under the laws of the United States of America or of any State
thereof, or any warrants to purchase or depositary shares or other rights in
respect of any such interests, and (y) all shares of stock, certificates,
certificated membership interests, instruments or other documents evidencing or
representing the same;

                                    (ii)     all right, title and interest of
Pledgor in and to all present and future payments, proceeds, dividends,
distributions, instruments, compensation, property, assets, interests and rights
in connection with or related to the collateral listed in clause (i) above, and
all monies due or to become due and payable to Pledgor in connection with or
related to such collateral or otherwise paid, issued or distributed from time to
time in respect of or in exchange therefor, and any certificate, instrument or
other document evidencing or representing the same (including, without
limitation, all proceeds of dissolution or liquidation);

                                    (iii)    all security entitlements in any
and all of the foregoing; and

                                    (iv)     all proceeds of any and all of the
foregoing, of every kind, and all proceeds of such proceeds.

                           (b) The shares of stock, certificates, certificated
membership interests, instruments or other documents evidencing or representing
the foregoing shall be collectively referred to herein as the "PLEDGED
SECURITIES".

                           (c)      As of the date of this Agreement, the
Pledged Collateral constitutes all of the equity interests of Pledgor in the
entities set out on Schedule 1.

                  2.03.    Perfection; Delivery of Certificates, Instruments,
Etc.

                           (a) Pledgor shall deliver to Secured Party or any
appointed Delegate:

                                    (i)      all Pledged Securities and all
other original shares of stock, certificates, certificated membership interests,
instruments and other documents evidencing or representing the Pledged
Collateral concurrently with the execution and delivery of this Agreement,
except that Pledgor may deliver the documents in respect of Sphere
Communications Inc., and Inviscid Networks, Inc., within sixty (60) days after
the date hereof; and

                                    (ii)     the original shares of stock,
certificates, certificated membership interests, instruments or other documents
evidencing or representing all Pledged Collateral (other than

                             STOCK PLEDGE AGREEMENT

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                                                                               4

Pledged Collateral that this Agreement specifically permits Pledgor to retain)
within ten (10) Business Days after Pledgor's receipt thereof.

                           (b) All Pledged Securities that are certificated
securities shall be in bearer form or, if in registered form, shall be endorsed
in blank pursuant to a stock power in the form of Exhibit A.

                           (c) With respect to uncertificated Pledged
Collateral, Pledgor shall either (i) cause the issuer to enter into a written
agreement or other authenticated record with Secured Party, in form and
substance satisfactory to Secured Party, pursuant to which such issuer shall
agree, among other things, to comply with instructions from Secured Party as to
such securities, without further consent of Pledgor or any nominee of Pledgor,
or (ii) arrange, at Secured Party's option, for either Secured Party or an
appointed Delegate to become the registered owner of the securities.

                           (d) With respect to Pledged Collateral held or
acquired by Pledgor or its nominee through a securities intermediary, Pledgor
shall either (i) cause such securities intermediary to enter into a written
agreement or other authenticated record with Secured Party, in form and
substance satisfactory to Secured Party, pursuant to which such securities
intermediary shall, among other things, agree to comply with entitlement orders
or other instructions from Secured Party to such securities intermediary as to
such Pledged Collateral, without further consent of Pledgor or any nominee of
Pledgor, or (ii) arrange, at Secured Party's option, for Secured Party or an
appointed Delegate to become the entitlement holder with respect to such Pledged
Collateral.

                  2.04.    Registration. At any time after an Enforcement Event
has occurred, Secured Party (acting on instructions received pursuant to the
terms of the STID) may cause all or any of the Pledged Securities to be
transferred to or registered in its name or the name of its nominee or nominees.

                  2.05.    Authorization; Responsibility.

                           (a) Pledgor hereby irrevocably authorizes Secured
Party at any time and from time to time to file in any jurisdiction in which the
UCC has been adopted any initial financing statements and amendments thereto
that (a) describe the Pledged Collateral and (b) contain any other information
required by part 5 of Article 9 of the UCC for the sufficiency or filing office
acceptance of any initial financing statement or amendment, including the type
of organization of Pledgor and any organization identification number issued to
Pledgor. Pledgor agrees to furnish any such information to Secured Party
promptly upon request. Pledgor also ratifies its authorization for Secured Party
to have filed in any UCC jurisdiction any like initial financing statements or
amendments thereto if filed prior to the date hereof. The authorization
conferred on Secured Party under this Section 2.05 is solely to protect Secured
Party's interest in the Pledged Collateral and shall not impose any duty on
Secured Party to act.

                  (b) Clause 14.8 of the STID is hereby incorporated herein by
this reference with the same effect as though set forth in its entirety herein.

         Section 3.    REPRESENTATIONS AND WARRANTIES OF PLEDGOR.

         Pledgor hereby represents and warrants to Secured Party that:

                             STOCK PLEDGE AGREEMENT

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                                                                               5

                  3.01.    Description of Pledged Securities. Set forth on
Schedule 1 is a complete and accurate list and description of all the Pledged
Securities. All information set forth on Schedule 1 is accurate and complete.

                  3.02.    Nature of Pledged Securities. The Pledged Securities
have been duly and validly issued and are fully paid and non-assessable,
provided that this Section shall apply only to Pledged Securities representing
equity interests in entities that are also Guarantors.

                  3.03.    Title to Pledged Collateral. Pledgor is the sole
legal and beneficial owner of the Pledged Collateral free and clear of any Liens
other than the Security Interests and Liens permitted under the Indentures,
provided that this Section shall apply only to Pledged Collateral constituting
equity interests in entities that are also Guarantors.

                  3.04.    Omitted.

         Section 4.    COVENANTS OF PLEDGOR.

                  4.01.    Pledgor's Name, Legal Status, Locations. Without
providing at least thirty (30) days' prior written notice to Secured Party,
Pledgor shall not change (a) its name; (b) its type of organization, its
jurisdiction of organization or other legal structure; or (c) its place of
business or (if it has more than one place of business) its chief executive
office or its mailing address.

                  4.02.    Pledgor's Organizational Number. Without providing at
least thirty (30) days' prior written notice to Secured Party, Pledgor shall not
change its organizational identification number if it has one. If Pledgor does
not have an organizational identification number and later obtains one, Pledgor
shall forthwith notify Secured Party of such organizational identification
number.

                  4.03.    Title to Collateral. Except for the Security Interest
herein granted, Pledgor shall be the owner of the Pledged Collateral free from
any Lien, except Liens not expressly prohibited under the Indentures.
Notwithstanding the foregoing, Pledgor shall be permitted to sell or otherwise
dispose of the Pledged Collateral if and to the extent permitted by the
Indentures.

                  4.04.    Further Assurances. Pledgor further agrees to take
any other action requested by Secured Party to ensure the attachment, perfection
and first priority of, and the ability of Secured Party to enforce, the Security
Interest created or purported to be created by or pursuant to this Agreement in,
any and all of the Pledged Collateral or for the exercise of rights and remedies
of Secured Party provided by or pursuant to this Agreement or by law including,
without limitation, executing, delivering and recording, as applicable, such
further instruments and documents and taking such further action as requested by
Secured Party.

         Section 5.    VOTING RIGHTS AND CERTAIN PAYMENTS PRIOR TO ENFORCEMENT
EVENT.

                  5.01.    Voting Rights and Ordinary Payments Prior to an
Enforcement Event. So long as no Enforcement Event shall have occurred and be
continuing, Pledgor shall be entitled:

                           (a) to exercise, as it shall think fit, in a manner
that would not create or result in a Default or an Event of Default under the
Indentures, the voting power with respect to the Pledged Collateral; and

                             STOCK PLEDGE AGREEMENT

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                                                                               6

                           (b) to receive and retain for its own account any and
all payments, proceeds, dividends, distributions, monies, compensation,
property, assets, instruments or rights to the extent such are permitted
pursuant to the terms of the Indentures.

                  5.02.    Voting Rights and Ordinary Payments After an
Enforcement Event. Upon the occurrence and during the continuance of any
Enforcement Event, all rights of Pledgor to exercise or refrain from exercising
the voting and other consensual rights that it would otherwise be entitled to
exercise pursuant to Section 5.01(a) hereof and to receive the payments,
proceeds, dividends, distributions, monies, compensation, property, assets,
instruments or rights that Pledgor would otherwise be authorized to receive and
retain pursuant to Section 5.01(b) hereof shall cease, and thereupon Secured
Party shall be entitled to exercise all voting power with respect to the Pledged
Securities and to receive and retain, as additional collateral hereunder, any
and all payments, proceeds, dividends, distributions, monies, compensation,
property, assets, instruments or rights at any time declared or paid upon any of
the Pledged Collateral during such an Enforcement Event and otherwise to act
with respect to the Pledged Collateral as outright owner thereof. Pledgor hereby
grants Secured Party and each Delegate an irrevocable proxy to exercise all
voting powers with respect to the Pledged Securities.

         Section 6.    ALL PAYMENTS IN TRUST. All payments, proceeds, dividends,
distributions, monies, compensation, property, assets, instruments or rights
that are received by Pledgor contrary to the provisions of Section 5 shall be
received and held in trust for the benefit of Secured Party, shall be segregated
by Pledgor from other funds of Pledgor and shall be forthwith paid over to
Secured Party as Pledged Collateral in the same form as so received (with any
necessary endorsement).

         Section 7.    EXPENSES.

                           (a) Pledgor shall, from time to time and promptly on
demand by Secured Party reimburse to Secured Party all costs and expenses
(including legal fees) on a full indemnity basis together with any applicable
VAT incurred by Secured Party (provided that in relation to clause (i) of this
Section 7(a), such costs and expenses must be properly incurred) in connection
with:

                                    (i)      the execution, release and
discharge of this Agreement and the Security created or intended to be created
in respect of the Pledged Collateral and perfection of the Security Interest
contemplated in this Agreement or in any such documents or forming part of the
Security Interest created or intended to be created in respect of the Pledged
Collateral;

                                    (ii)     the actual or contemplated
exercise, preservation and/or enforcement of any of the rights, powers and
remedies of, or the performance of the duties and obligations of, Secured Party,
or any amendment or waiver in respect of this Agreement;

                                    (iii)    the foreclosure of any Pledged
Collateral; and

                                    (iv)     the preservation and/or enforcement
of the Security created or intended to be created in respect of the Pledged
Collateral;

the amount of which costs and expenses shall carry interest from the date of
such demand until so reimbursed at the rate and on the basis set forth in clause
18.4.1 of the STID.

                             STOCK PLEDGE AGREEMENT

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                                                                               7

                           (b) Pledgor shall pay, promptly on demand of Secured
Party all stamp, registration, notarial and other similar Taxes or fees paid or
payable by Secured Party in connection with any action taken or contemplated by
or on behalf of Secured Party for perfecting, enforcing, releasing, cancelling,
reassigning or resolving any doubt concerning, or for any other purpose in
relation to this Agreement, any amendment thereto, any transfer and/or
assignment of the rights and/or obligations under the same or the Security
Interest created or intended to be created in respect of the Pledged Collateral
and shall, from time to time, indemnify Secured Party promptly on demand against
any liabilities, costs, claims and expenses resulting from any failure to pay by
Pledgor or any delay by Pledgor in paying any such Taxes or fees.

         Section 8.    REMEDIES.

                  8.01.    Disposition Upon Enforcement Event and Related
Provisions.

                           (a) Upon the occurrence and during the continuance of
any Enforcement Event, the Security Interests shall be immediately enforceable
and the Secured Party (acting on instructions received pursuant to the terms of
the STID) may exercise in respect of the Pledged Collateral, in addition to
other rights and remedies provided for herein, in the STID or otherwise
available to Secured Party, all rights of voting, exercise and conversion with
respect to the Pledged Collateral and all of the rights and remedies of a
secured party on default under the NYUCC at that time (whether or not applicable
to the affected Pledged Collateral) and may also, without obligation to resort
to other security, at any time and from time to time sell, resell, assign and
deliver, in its sole discretion, all or any of the Pledged Collateral, in one or
more parcels at the same or different times, and all right, title and interest,
claim and demand therein and right of redemption thereof, on any securities
exchange on which any Pledged Collateral may be listed, or at public or private
sale, for cash, upon credit or for future delivery, and in connection therewith
Secured Party may grant options.

                           (b)      If any of the Pledged Collateral is sold by
Secured Party upon credit or for future delivery, Secured Party shall not be
liable for the failure of the purchaser to purchase or pay for the same and, in
the event of any such failure, Secured Party may resell such Pledged Collateral.
In no event shall Pledgor be credited with any part of the proceeds of sale of
any Pledged Collateral until cash payment therefor has actually been received by
Secured Party.

                           (c)      Secured Party may purchase or sell any
Pledged Collateral at any public sale and, if any Pledged Collateral is of a
type customarily sold in a recognized market or is of the type that is the
subject of widely distributed standard price quotations, Secured Party may
purchase or sell such Pledged Collateral at private sale, and in each case may
make payment therefor by any means, including, without limitation, by release or
discharge of Secured Obligations in lieu of cash payment.

                           (d)      Pledgor recognizes that Secured Party may be
unable to effect a public sale of all or part of the Pledged Collateral
consisting of securities by reason of certain prohibitions contained in the
Securities Act of 1933, as amended (the "SECURITIES ACT"), or in applicable Blue
Sky or other state securities laws, as now or hereafter in effect, but may be
compelled to resort to one or more private sales to a restricted group of
purchasers who will be obliged to agree, among other things, to acquire such
securities for their own account, for investment and not with a view to the
distribution or resale thereof. Pledgor agrees that any such Pledged Collateral
sold at any such private sale may be sold at a price and

                             STOCK PLEDGE AGREEMENT

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upon other terms less favorable to the seller than if sold at public sale and
that each such private sale shall be deemed to have been made in a commercially
reasonable manner. Secured Party shall have no obligation to delay the sale of
any such securities for the period of time necessary to permit the issuer of
such securities, even if such issuer would agree, to register such securities
for public sale under the Securities Act. Pledgor agrees that private sales made
under the foregoing circumstances shall be deemed to have been made in a
commercially reasonable manner.

                           (e)      No demand, advertisement or notice, all of
which are hereby expressly waived, shall be required in connection with any sale
or other disposition of any part of the Pledged Collateral that threatens to
decline speedily in value or that is of a type customarily sold on a recognized
market; otherwise Secured Party shall give Pledgor at least ten (10) days' prior
notice of the time and place of any public sale and of the time after which any
private sale or other disposition is to be made, which notice Pledgor agrees is
commercially reasonable.

                           (f)      Secured Party shall not be obligated to make
any sale of Pledged Collateral if it shall determine not to do so, regardless of
the fact that notice of sale may have been given. Secured Party may, without
notice or publication, adjourn any public or private sale or cause the same to
be adjourned from time to time by announcement at the time and place fixed for
sale, and such sale may, without further notice, be made at the time and place
to which the same was so adjourned.

                           (g)      The remedies provided herein in favor of
Secured Party shall not be deemed exclusive, but shall be cumulative, and shall
be in addition to all other remedies in favor of Secured Party existing at law
or in equity.

                           (h)      To the extent that applicable law imposes
duties on Secured Party to exercise remedies in a commercially reasonable
manner, Pledgor acknowledges and agrees that it is not commercially unreasonable
for Secured Party (i) to advertise dispositions of Pledged Collateral through
publications or media of general circulation; (ii) to contact other persons,
whether or not in the same business as Pledgor, for expressions of interest in
acquiring all or any portion of the Pledged Collateral; (iii) to hire one or
more professional auctioneers to assist in the disposition of Pledged
Collateral; (iv) to dispose of Pledged Collateral by utilizing Internet sites
that provide for the auction of assets of the types included in the Pledged
Collateral or that have the reasonable capability of doing so, or that match
buyers and sellers of assets; (v) to disclaim disposition warranties, or (vi) to
the extent deemed appropriate by Secured Party, to obtain the services of
brokers, investment bankers, consultants and other professionals to assist
Secured Party in the disposition of any of the Pledged Collateral. Pledgor
acknowledges that the purpose of this clause (h) is to provide non-exhaustive
indications of what actions or omissions by Secured Party would not be
commercially unreasonable in Secured Party's exercise of remedies against the
Pledged Collateral and that other actions or omissions by Secured Party shall
not be deemed commercially unreasonable solely on account of not being indicated
in this clause (h). Without limiting the foregoing, nothing contained in this
clause (h) shall be construed to grant any rights to Pledgor or to impose any
duties on Secured Party that would not have been granted or imposed by this
Agreement or by applicable law in the absence of this clause (h).

                  8.02.    Secured Party Appointed Attorney-in-Fact

                             STOCK PLEDGE AGREEMENT

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                                                                               9

                           (a) To effectuate the terms and provisions hereof,
Pledgor hereby appoints Secured Party as Pledgor's attorney-in-fact for the
purpose of carrying out the provisions of this Agreement and taking any action
and executing any instrument that Secured Party from time to time in Secured
Party's reasonable discretion may deem necessary or advisable to accomplish the
purposes of this Agreement. Without limiting the generality of the foregoing,
Secured Party shall, from and after the occurrence and during the continuance of
an Enforcement Event, at the expense of Pledgor, have the right and power to:

                                    (i)      receive, endorse and collect all
checks and other orders for the payment of money made payable to Pledgor
representing any interest or dividend or other distribution or amount payable in
respect of the Pledged Collateral or any part thereof and to give full discharge
for the same;

                                    (ii)     execute endorsements, assignments
or other instruments of conveyance or transfer with respect to all or any of the
Pledged Collateral;

                                    (iii)    exercise all rights of Pledgor as
owner of the Pledged Collateral including, without limitation, the right to sign
any and all amendments, instruments, certificates, proxies, and other writings
necessary or advisable to exercise all rights and privileges of (or on behalf
of) the owner of the Pledged Collateral, including, without limitation, all
voting rights with respect to the Pledged Securities;

                                    (iv)     ask, demand, collect, sue for,
recover, compound, receive and give acquittance and receipts for moneys due and
to become due under or in respect of any of the Pledged Collateral;

                                    (v)      file any claims or take any action
or institute any proceedings that Secured Party may deem necessary or desirable
for the collection of any of the Pledged Collateral or otherwise to enforce the
rights of Secured Party with respect to any of the Pledged Collateral; and

                                    (vi)     generally to sell, transfer,
pledge, make any agreement with respect to or otherwise deal with any of the
Pledged Collateral as fully and completely as though Secured Party were the
absolute owner thereof for all purposes, and to do, at Secured Party's option
and Pledgor's expense, at any time or from time to time, all acts and things
that Secured Party deems reasonably necessary to protect, preserve or realize
upon the Pledged Collateral.

                           (b) Pledgor hereby ratifies and approves all acts of
Secured Party made or taken pursuant to this Section 8.02. Neither Secured Party
nor any person designated by Secured Party shall be liable for any acts or
omissions or for any error of judgment or mistake of fact or law, except such as
may result from Secured Party's gross negligence or willful misconduct. This
power, being coupled with an interest, is irrevocable so long as this Agreement
shall remain in force.

                  8.03.    Secured Party's Duties of Reasonable Care

                           (a) Secured Party's sole duty with respect to the
custody, safekeeping and physical preservation of the Collateral in its
possession, under Section 9-207 of the NYUCC or otherwise, shall be to deal with
it in the same manner as Secured Party deals with similar property for its own
account.

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              10

                           (b) Pledgor waives any restriction or obligation
imposed on Secured Party under Sections 9-207(c)(1) and 9-207(c)(2) of the
NYUCC.

                  8.04.    Indemnification. Clause 19 of the STID is hereby
incorporated herein by this reference with the same effect as though set forth
in its entirety herein, as if Pledgor were "Issuer" under such clause.

                  8.05.    Prior Recourse. Secured Party's prior recourse to any
Pledged Collateral shall not constitute a condition of any demand, suit or
proceeding for payment or collection of the Secured Obligations.

                  8.06.    Secured Party May Perform. If Pledgor fails to
perform any agreement contained herein, Secured Party may itself perform or
cause performance of such agreement, and the expenses of Secured Party incurred
in connection therewith shall be treated as provided in Section 7 hereof.

         Section 9.    SURETYSHIP WAIVERS BY PLEDGOR; OBLIGATIONS ABSOLUTE.

                           (a) Pledgor waives demand, notice, protest, notice of
acceptance of this Agreement, notice of loans made, credit extended, collateral
received or delivered or other action taken in reliance hereon and all other
demands and notices of any description thereof, all in such manner and at such
time or times as Secured Party may deem advisable. Secured Party shall have no
duty as to the collection or protection of the Pledged Collateral or any income
thereon, nor as to the preservation of rights against prior parties, nor as to
the preservation of any rights pertaining thereto beyond the safe custody
thereof as set forth in Section 8.03.

                           (b) All rights of Secured Party hereunder, the
Security Interests and all obligations of Pledgor hereunder shall be absolute
and unconditional irrespective of (i) any lack of validity or enforceability of
the STID, any other Relevant Document, any agreement with respect to any of the
Secured Obligations or any other agreement or instrument relating to any of the
foregoing, (ii) any change in the time, manner or place of payment of, or in any
other term of, all or any of the Secured Obligations, or any other amendment or
waiver of or any consent to any departure from the STID, any other Relevant
Document, or any other agreement or instrument, (iii) any exchange, release or
non-perfection of any Lien on other collateral, or any release or amendment or
waiver of or consent under or departure from or any acceptance of partial
payment thereon and or settlement, compromise or adjustment of any of the
Secured Obligations or of any guarantee, securing or guaranteeing all or any of
the Secured Obligations, or (iv) any other circumstance that might otherwise
constitute a defense available to, or a discharge of, Pledgor in respect of the
Secured Obligations or this Agreement.

         Section 10.   MARSHALLING. Secured Party shall not be required to
marshal any present or future collateral security (including but not limited to
this Agreement and the Pledged Collateral) for, or other assurances of payment
of, the Secured Obligations or any of them or to resort to such collateral
security or other assurances of payment in any particular order, and all of its
rights hereunder and in respect of such collateral security and other assurances
of payment shall be cumulative and in addition to all other rights, however
existing or arising. To the extent that it lawfully may, Pledgor hereby agrees
that it shall not invoke any law relating to the marshalling of collateral that
might cause delay in or impede the enforcement of Secured Party's rights under
this Agreement or under any other instrument creating or evidencing any of the
Secured Obligations or under which any of the Secured Obligations is outstanding

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              11

or by which any of the Secured Obligations is secured or payment thereof is
otherwise assured, and, to the extent that it lawfully may, Pledgor hereby
irrevocably waives the benefits of all such laws.

         Section 11.   PROCEEDS OF DISPOSITIONS. After deducting all expenses
payable to Secured Party, including, without limitation, pursuant to Section 7,
and after making any payments required by Sections 9-608(a)(1)(A) or 9-615(a)(1)
of the NYUCC, the residue of any proceeds of collection or sale of the Secured
Obligations or Pledged Collateral shall, to the extent actually received in
cash, be applied to the payment of the remaining Secured Obligations in such
order or preference as is provided in the STID, proper allowance and provision
being made for any Secured Obligations not then due or held as additional
Pledged Collateral. Pledgor shall remain liable for any deficiency in the
payment of the Secured Obligations.

         Section 12.   TERMINATION. Secured Party shall, at the cost of Pledgor,
release the Lien and cancel the security constituted by this Agreement and
procure the reassignment (to the extent necessary) to Pledgor of the Pledged
Collateral in accordance with the terms and subject to the conditions and
circumstances set out in clause 5.3 and clause 5.4 of the STID and without
recourse to, or any representations or warranty by, Secured Party or any of its
nominees.

         Section 13.   REINSTATEMENT. The obligations of Pledgor pursuant to
this Agreement shall continue to be effective or automatically be reinstated, as
the case may be, if at any time payment of any of the Secured Obligations is
rescinded or otherwise must be restored or returned by Secured Party upon the
insolvency, bankruptcy, dissolution, liquidation or reorganization of Pledgor or
any other obligor or otherwise, all as though such payment had not been made.

         Section 14.   MISCELLANEOUS.

                  14.01.   Notices. Except as otherwise provided herein,
whenever it is provided herein that any notice, demand, request, consent,
approval, declaration or other communication shall or may be given to or served
upon any of the parties by any other party, or whenever any of the parties
desires to give and serve upon any other party any communication with respect to
this Agreement, each such notice, demand, request, consent, approval,
declaration or other communication shall be in writing and shall be given in the
manner and to the address, and deemed received, as provided for in clause 20 of
the STID.

                  14.02.   GOVERNING LAW; CONSENT TO JURISDICTION. THIS
AGREEMENT SHALL BE GOVERNED BY, AND SHALL BE CONSTRUED AND ENFORCED IN
ACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO
CONFLICTS OF LAWS PRINCIPLES (WHICH PRINCIPLES SHALL BE DEEMED TO EXCLUDE
SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW). Pledgor agrees that any
suit for the enforcement of this Agreement may be brought in the courts of the
State of New York or any federal court sitting therein and consents to the
non-exclusive jurisdiction of such court and to service of process in any such
suit being made upon Pledgor by mail at the address set forth below its name on
the signature pages hereof. Pledgor hereby waives any objection that it may now
or hereafter have to the venue of any such suit or any such court or that such
suit is brought in an inconvenient court.

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              12

                  14.03.   WAIVER OF JURY TRIAL, ETC. PLEDGOR WAIVES ITS RIGHT
TO A JURY TRIAL WITH RESPECT TO ANY ACTION OR CLAIM ARISING OUT OF ANY
LITIGATION OR DISPUTE DIRECTLY OR INDIRECTLY ARISING OUT OF, UNDER OR IN
CONNECTION WITH THIS AGREEMENT, ANY RIGHTS OR OBLIGATIONS HEREUNDER OR THE
PERFORMANCE OF ANY SUCH RIGHTS OR OBLIGATIONS. Pledgor certifies that neither
Secured Party nor any representative, agent or attorney of Secured Party has
represented, expressly or otherwise, that Secured Party would not, in the event
of litigation, seek to enforce the foregoing waivers and acknowledges that, in
entering into the STID and the other Relevant Documents to which Secured Party
is a party, Secured Party is relying upon, among other things, the waivers and
certifications contained in this Section 14.03.

                  14.04.   Counterparts. This Agreement may be executed in two
or more separate counterparts, each of which shall constitute an original and
all of which shall collectively and separately constitute one and the same
agreement.

                  14.05.   Headings. The headings of each section of this
Agreement are for convenience only and shall not define or limit the provisions
thereof.

                  14.06.   Severability. In the event any one or more of the
provisions contained in this Agreement should be held invalid, illegal or
unenforceable in any respect, the validity, legality and enforceability of the
remaining provisions contained herein shall not in any way be affected or
impaired thereby (it being understood that the invalidity of a particular
provision in a particular jurisdiction shall not in and of itself affect the
validity of such provision in any other jurisdiction).

                  14.07.   Survival of Agreement. All covenants, agreements,
representations and warranties made by Pledgor herein and in the certificates or
other instruments prepared or delivered in connection with or pursuant to this
Agreement shall be considered to have been relied upon by Secured Party and
shall survive the execution and delivery of the Indentures and the other
Relevant Documents, regardless of any investigation made by Secured Party, and
shall continue in full force and effect until this Agreement shall terminate (or
thereafter to the extent provided herein).

                  14.08.   Matters Relating to Secured Party. Each of Clauses
14, 15, 16, 18 and 19 of the STID is hereby incorporated herein by this
reference with the same effect as though set forth in its entirety herein.

                  14.09.   Binding Effect; Several Agreement. This Agreement is
binding upon Pledgor and Secured Party and their respective successors and
permitted assigns, and shall inure to the benefit of Pledgor, Secured Party and
their respective successors and permitted assigns, except that Pledgor shall
have no right to assign or transfer its rights or obligations hereunder or any
interest herein (and any such assignment or transfer shall be void) except as
expressly contemplated by this Agreement or the Indentures. Secured Party may
assign and transfer all or any of its rights and obligations under this
Agreement in accordance with the STID, and Secured Party shall be entitled to
disclose such information concerning the Pledgor and this Agreement as the
Secured Party considers appropriate to any actual or proposed direct or indirect
successor or to any person to whom information may be required to be disclosed
by applicable law.

                  14.10.   Waivers; Amendment.

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              13

                           (a) No failure or delay of Secured Party in
exercising any power or right hereunder shall operate as a waiver thereof, nor
shall any single or partial exercise of any such right or power, or any
abandonment or discontinuance of steps to enforce such a right or power,
preclude any other or further exercise thereof or the exercise of any other
right or power. The rights and remedies of Secured Party hereunder and under the
STID are cumulative and are not exclusive of any rights or remedies that Secured
Party would otherwise have. No waiver of any provisions of this Agreement or
consent to any departure by Pledgor therefrom shall in any event be effective
unless the same shall be permitted by paragraph (b) below, and then such waiver
or consent shall be effective only in the specific instance and for the purpose
for which given. No notice to or demand on Pledgor in any case shall entitle
Pledgor to any other or further notice or demand in similar or other
circumstances.

                           (b) Neither this Agreement nor any provision hereof
may be waived, amended or modified except pursuant to an agreement or agreements
in writing entered into by Secured Party and Pledgor, subject to any consent
required in accordance with the STID.

                  14.11.   Omitted.

                 [the rest of this page is intentionally blank]

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              14

IN WITNESS WHEREOF, intending to be legally bound, Pledgor has caused this Stock
Pledge Agreement to be duly executed as of the date first above written.

MARCONI COMMUNICATIONS, INC.

By: ___________________________
    Name: T.C.R SHEPHERD
    Title: AUTHORISED SIGNATORY

Address for New York service of process:

Marconi Electronic Systems Holdings Inc.
c/o Marconi Communications, Inc.
333 Pierce Road - Suite 370
Itasca, Illinois 60143 USA
Attention: Patricia Hoffman
Telecopier No.: (630) 285-1514

ACCEPTED AND AGREED:

THE LAW DEBENTURE TRUST CORPORATION p.l.c.
as Secured Party

By: ___________________________
    Name: R.D RANCE
    Title: DIRECTOR

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              15

                                                                   SCHEDULE 1 TO
                                                          STOCK PLEDGE AGREEMENT

                   LIST AND DESCRIPTION OF PLEDGED SECURITIES

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              16

                          MARCONI COMMUNICATIONS, INC.

Description of Pledged Securities:

<TABLE>
<CAPTION>
                                                                                                                  Percentage
              Issuer                          Class                                        Number                  of total
             of Stock                        of Stock          Certificate Numbers        of Shares                 shares
------------------------------------   ---------------------   -------------------   --------------------   -----------------------
<S>                                    <C>                     <C>                   <C>                    <C>
Marconi Networks Worldwide, Inc.       Common                           1                    100                     100

Marconi Communications Technology,     Common                           1                    100                     100
Inc.

Marconi Communications Federal,        Common                           1                    100                     100
Inc.

Marconi Communications C.A., Inc.      Common                           3                    100                     100

Nemesys Holding Company                Common                           2                    985                     100

Custom Telecom Contractors, Inc.       Common                           3                    500                     100
(Missouri)

Marconi Acquisition Corp.              Common                           2                    100                     100

ALANTEC International Inc.             Common                           4                  10,000                    100
(California)

Marconi Intellectual Property          Common                           1                    100                     100
(Ringfence) Inc.

Gnome, Inc.                            Common                           3                   1,000                     90

Inviscid Networks, Inc.                Common                                                                       19.9%

Sphere Communications Inc.             Common                                                                         8$
</TABLE>

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              17

Exact Name of Pledgor: Marconi Communications, Inc.

Pledgor is a corporation.

Pledgor is organized under the laws of Delaware.

[Pledgor's organizational identification number: 2316994

Mailing address of Pledgor:

3000 Marconi Drive
Warrendale, PA 15086

CHECK AND COMPLETE ONE OF THE FOLLOWING:

[X] Pledgor's place of business is the same as its mailing address.

[ ] Pledgor's place of business is different from its mailing address, and the
address of Pledgor's place of business is as follows:

CHECK AND COMPLETE ONE OF THE FOLLOWING:

[X] Pledgor has only one place of business.

[ ] Pledgor's has more than one place of business, and the address of Pledgor's
chief executive office is as follows:

                             STOCK PLEDGE AGREEMENT

<PAGE>

                                                                              18

                                                                    EXHIBIT A TO
                                                          STOCK PLEDGE AGREEMENT

                               FORM OF STOCK POWER

                                   STOCK POWER

FOR VALUE RECEIVED, [transferring shareholder] hereby sells, assigns and
transfers unto_______________________________________________________________(1)
[number] shares of the Common Stock, par value $[ ] per share, of [Name of
Issuer], a [jurisdiction] corporation (the "Corporation") standing in the name
of ____________________________ on the books of the Corporation represented by
Certificate(s) No. ___________ herewith and does hereby irrevocably constitute
and appoint ___________________________________ as attorney to transfer the said
shares on the books of the Corporation with full power of substitution in the
premises.

Dated: ____________________________

                                              By: ______________________________

-------------------
1        When this executed stock power is delivered in connection with the
         notice described in Section 5, neither the name of the transferee, the
         name of the attorney nor the date should be filled in. Only the number
         of shares of the Common Stock, the par value, the name of the Issuer,
         its jurisdiction and the certificate number(s) should be filled in, and
         the stock power should be signed by someone with signing authority for
         the transaction.

                             STOCK PLEDGE AGREEMENT