1
 
                                                                     EXHIBIT 5.1
 
                                                                    June 6, 1997
 
Trenwick Group Inc.
Metro Center
One Station Place
Stamford, Connecticut 06902
 
     Re:  Trenwick Group Inc.
        Registration Statement on Form S-4
        Filed With the Securities and Exchange
        Commission on June 6, 1997
 
Ladies and Gentlemen:
 
     We have acted as special counsel to Trenwick Group Inc., a Delaware
corporation (the "Company"), in connection with the registration by the Company
and Trenwick Capital Trust I, a statutory business trust formed under the laws
of the State of Delaware (the "Trust") of (i) one hundred ten thousand (110,000)
8.82% Exchange Subordinated Capital Income Securities (Liquidation Amount $1,000
per Capital Security) of the Trust (the "Capital Securities"), (ii) $110,000,000
in aggregate principal amount of 8.82% Exchange Junior Subordinated Deferrable
Interest Debentures due February 1, 2037 (the "Junior Subordinated Debentures")
to be issued under the Indenture dated as of January 31, 1997 (the "Indenture")
between the Company and The Chase Manhattan Bank, as trustee, and (iii) the
Exchange Capital Securities Guarantee Agreement to be entered into between the
Company as guarantor, and The Chase Manhattan Bank, as trustee (the "Capital
Securities Guarantee," together, with the Capital Securities and the Junior
Subordinated Debentures, the "Securities"), under the Registration Statement on
Form S-4, as filed with the Securities and Exchange Commission on the date
hereof (the "Registration Statement").
 
     We are familiar with the proceedings of the Company relating to the
authorization of the Securities and the Indenture filed as an Exhibit to the
Registration Statement. In addition, we have made such further examinations of
law and fact as we have deemed appropriate in connection with the opinion
hereinafter set forth.
 
     Based on the foregoing and subject to the limitations, qualifications,
exceptions and assumptions set forth herein, we are of the opinion that:
 
          (i) The execution and delivery of the Capital Securities Guarantee
     have been duly authorized by all necessary corporate action of the Company
     and, when the Capital Securities Guarantee is authorized, executed and
     delivered by all other parties thereto in accordance with the terms
     thereof, will constitute a valid and binding obligation of the Company,
     enforceable against the Company in accordance with its terms, except as
     enforcement thereof may be limited by bankruptcy, insolvency,
     reorganization, fraudulent conveyance, moratorium or other similar laws
     relating to or affecting creditors' rights generally or by general
     equitable principles (regardless of whether considered in a proceeding in
     equity or at law).
 
          (ii) The execution and delivery of the Junior Subordinated Debentures
     have been duly authorized by all necessary corporate action of the Company,
     and the Junior Subordinated Debentures, when executed, authenticated and
     delivered in accordance with the Indenture, will be entitled to the
     benefits of the Indenture, and will constitute the valid and binding
     obligations of the Company, enforceable against the Company in accordance
     with their terms, except as enforcement thereof may be limited by
     bankruptcy, insolvency, reorganization, fraudulent conveyance, moratorium
     or other similar laws relating to or affecting creditors' rights generally
     or by general equitable principles (regardless of whether considered in a
     proceeding in equity or at law).
 
     We express no opinion as to any laws other than the Federal laws of the
United States, the laws of the State of New York and the General Corporation Law
of the State of Delaware.
 
     We hereby consent to the filing of this opinion as an Exhibit to the
Registration Statement and to the references to our firm as set forth under the
caption "Legal Matters" in the Prospectus constituting part of the Registration
Statement. In giving such consent, we do not hereby admit that we are within the
category of persons whose consent is required by Section 7 of the Securities Act
of 1933, as amended, and the rules and regulations thereunder.
 
                                          Very truly yours,
 
                                                 /s/ BAKER & MCKENZIE