Moody's has placed the B- ratings of Lloyd's syndicates 839 and 44 under review for a possible downgrade.
The ratings agency stressed that the Trenwick Managing Agents' syndicates are in good shape.
Syndicate 839's 2001 account performance is expected to be show a "significant" improvement from 1999, with a small loss of 2.5% currently estimated.
A Moody's statement said: "However, despite the progress made by Michael Watson and his management team, there is currently uncertainty over the extent and nature of the syndicate's capital backing for 2003.
"Trenwick are still in negotiations with banks in respect of the renewal / replacement of the $230m LOC facility which currently supports their Lloyd's underwriting. Furthermore, other sources of capital support for syndicate 839's 2003 underwriting are still being explored."
Moody's added that the uncertainty meant its rating has been placed under review for possible downgrade.