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                                                                     EXHIBIT 8.3

                      [LETTERHEAD OF MAYER, BROWN & PLATT]

                                August 23, 2000

LaSalle Re Holdings Limited
Continental Building
25 Church Street
Hamilton, HM 12, Bermuda

Ladies and Gentlemen:

     We have acted as your counsel in connection with: (i) the proposed schemes
of arrangement (the "Schemes") of LaSalle Re Holdings Limited ("LaSalle") and
LaSalle Re Limited ("LaSalle Re"), pursuant to which existing common
shareholders of LaSalle and holders of exchangeable non-voting common shares of
LaSalle Re (other than LaSalle) will receive common shares of Trenwick Group
Ltd. ("TGL") in exchange for their LaSalle and LaSalle Re shares; (ii) the
simultaneous proposed reorganization of Trenwick Group Inc. (the
"Reorganization") in which TGL will acquire the assets of Trenwick Group Inc.;
and (iii) the preparation and filing of the related Joint Proxy
Statement/Prospectus (the "Proxy Statement") and the Registration Statement on
Form S-4 of which the Proxy Statement forms a part (the "Registration
Statement"), filed with the Securities and Exchange Commission under the
Securities Exchange Act of 1934, as amended. Capitalized terms used but not
defined herein shall have the meaning given to such terms in the Proxy
Statement.

     Pursuant to the Schemes, the common shareholders of LaSalle and the holders
of exchangeable non-voting common of LaSalle Re (collectively, the "Exchanging
Shareholders") will receive one TGL common share (a "TGL Share") (subject to
adjustments) in exchange for each LaSalle common share or exchangeable
non-voting common share of LaSalle Re. TGL will not issue fractional TGL common
shares. Therefore, the Exchanging Shareholders will receive cash with respect to
any fractional shares of TGL due them.

     You have requested our opinion concerning certain United States federal
income tax consequences of the Schemes and Reorganization.

     In providing this opinion, we have relied on and assumed the accuracy of
(without any independent investigation or review thereof): (i) the description
of the Schemes and Reorganization as set forth in the Schemes and the exhibits
thereto, including the representations and covenants of LaSalle, LaSalle Re and
TGL contained therein; (ii) the description of the Schemes and Reorganization as
set forth in the Proxy Statement and the exhibits thereto; and (iii)
representations provided by LaSalle, LaSalle Re and TGL concerning certain facts
underlying and relating to the Schemes and Reorganization.

     In addition, we have assumed that: (i) the Schemes and Reorganization will
be reported by LaSalle, LaSalle Re and TGL in a manner consistent with the
opinion set forth below; (ii) any representation or statement that is
anticipated to be true or that is made "to the best of knowledge" or is
similarly qualified is correct without such qualification; (iii) the Schemes
will be valid under the laws of Bermuda; (iv) as to all matters as to which any
person or entity represents that it is not a party to, does not have, or is not
aware of any plan, intention, understanding or agreement, there is in fact no
such plan, intention, understanding or agreement; and (v) officers of LaSalle,
LaSalle Re and TGL who have signed the representations on behalf of those
respective entities are knowledgeable concerning the matters and are authorized
to make all of the representations set forth therein.

     For purposes of this opinion, a "U.S. Holder" is a beneficial owner of
common shares of LaSalle or exchangeable non-voting common of LaSalle Re
(collectively, the "LaSalle Shares") who is: (i) a citizen or resident of the
United States for federal income tax purposes; (ii) a corporation (or other
entity taxable as a corporation) or a partnership, organized under the laws of
the United States or any state thereof; (iii) an estate the income of which is
subject to United States federal income taxation regardless of source; or (iv) a
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trust if a United States court is able to exercise primary supervision over the
administration of such trust and one or more United States persons have the
authority to control all substantial decisions of such trust.

     To the extent that this opinion addresses the federal income tax
consequences of the Schemes to Exchanging Shareholders, this opinion only
addresses such consequences to Exchanging Shareholders who hold their LaSalle
Shares as capital assets within the meaning of Section 1221 of the Internal
Revenue Code of 1986, as amended (the "Code"). In addition, the conclusions set
forth in this opinion may not be fully applicable to shareholders subject to
special treatment under United States federal income tax law, such as financial
institutions, insurance companies, tax-exempt entities, dealers in securities,
certain United States expatriates, persons holding LaSalle Shares as part of an
integrated investment, U.S. holders whose functional currency is not the U.S.
dollar, non-U.S. Holders, shareholders who hold LaSalle Shares through a pass-
through entity, and shareholders who acquired LaSalle Shares through exercise of
employee stock options or otherwise as compensation.

     This opinion is based on current provisions of the Code, the Treasury
regulations promulgated thereunder, the Bermuda-U.S. Convention for the
Elimination of Double Taxation (the "Tax Treaty") and the interpretation of the
Code, such regulations and the Tax Treaty by the courts and the Internal Revenue
Service (the "IRS"), as they are in effect and exist at the date of this
opinion. It should be noted that statutes, regulations, judicial decisions and
administrative interpretations are subject to change at any time and, in some
circumstances, with retroactive effect. A material change that is made after the
date hereof in any of the foregoing bases for our opinion could adversely affect
our conclusion.

     Based upon and subject to the foregoing, it is our opinion that, for U.S.
federal income tax purposes:

          (1) The Schemes and Reorganization, taken together, will constitute a
     corporate organization described in Section 351 of the Code;

          (2) Neither LaSalle nor LaSalle Re will recognize income, gain or loss
     as a result of the Schemes and Reorganization;

          (3) A U.S. Holder owning 5% or more of the total shares of TGL by vote
     or value after the Schemes and Reorganization (i.e., a "Five Percent
     Shareholder") will be required under Section 367(a) of the Code to
     recognize gain (to the extent that the fair market value of the TGL Shares
     received exceeds the basis of LaSalle Shares surrendered therefor) but not
     loss upon the exchange of LaSalle Shares for TGL shares, unless such Five
     Percent Shareholder enters into a valid gain recognition agreement with the
     Internal Revenue Service;

          (4) A U.S. Holder owning less than 5% of the total shares of TGL by
     vote or value after the Schemes and Reorganization (i.e., an Exchanging
     Shareholder that is not a Five Percent Shareholder) will recognize no gain
     or loss upon the exchange of LaSalle Shares for TGL shares, other than with
     respect to cash in lieu of fractional shares as described below;

          (5) A U.S. Holder's basis in the TGL Shares received will equal the
     U.S. Holder's basis in its LaSalle Shares exchanged therefor, increased by
     any gain recognized (in the case of a Five-Percent Shareholder not entering
     into a valid gain recognition agreement) with respect to the exchange;

          (6) Other than with respect to a Five Percent Shareholder not entering
     into a gain recognition agreement, a U.S. Holder's holding period in its
     TGL Shares will include such shareholder's holding period in the LaSalle
     Shares exchanged therefor;

          (7) A U.S. Holder receiving cash in lieu of fractional shares of TGL
     will be treated as if such holder received the fractional TGL share in the
     Schemes and then exchanged the fractional share for cash in a redemption by
     TGL;

          (8) A U.S. Holder receiving cash in lieu of a fractional TGL share
     generally will recognize gain or loss equal to the cash amount received for
     the fractional TGL share reduced by the portion of the holder's tax basis
     in the LaSalle Shares that is allocable to the fractional TGL share; and
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          (9) Gain or loss from the receipt of cash in lieu of a fractional TGL
     share will be long-term capital gain or loss if the U.S. Holder's holding
     period in the fractional TGL share (taking into account the holding period
     of the LaSalle Shares exchanged in the Schemes) is more than one year.

     This opinion is rendered to you in connection with the Schemes and
Reorganization. This opinion may not be used or relied upon by any other person
or for any other purpose without our prior written consent. We consent, however,
to the filing of this opinion as an exhibit to the Registration Statement and to
the use of our name under the captions "Material Income Tax Consequences of the
Transaction" and "Legal Matters" in the Proxy Statement.

     This opinion is based on facts and circumstances existing on the date
hereof, and assumes no change in material facts prior to the closing date. The
opinion is further conditioned on the representations of LaSalle, LaSalle Re and
TGL being reconfirmed as of the closing date.

                                          Sincerely,

                                          /s/ GEORGE W. CRAVEN
                                          --------------------------------------
                                               MAYER, BROWN & PLATT