Lloyd's and Ace are facing the threat of rating downgrades as a result of their exposures to the unparalleled losses from Hurricane Katrina.
Standard and Poor's (S&P) has placed 10 insurance and reinsurance groups on creditwatch with negative implications including Ace, Lloyd's and Swiss Re.
Financial strength rating is a measure of an insurance company's ability to pay claims. A negative credit watch means that the rating may be lowered.
S&P currently rates the financial strength for Lloyd's at A and A- for Ace. S&P's highest rating is AAA.
But S&P said downgrades were not "inevitable".
A Lloyd's spokeswoman said: "The credit watch is