Rating agency Standard & Poor's has lowered the outlook on Swiss Re's credit rating to 'negative' from 'stable'.
Standard & Poor's (S&P) said the move was prompted after the company revealed it had posted a loss for 2002.
S&P said it has affirmed Swiss Re's AA+/A-1 counterparty credit and AA+ financial strength ratings.
An S&P statement said: "The outlook revision reflects the weaker than expected 2003 renewals and that expected profits for 2002 will fall short of Standard & Poor's expectations."
"Swiss Re's ratings may be cut if it fails to maintain a combined ratio below 100% for an extended period of time on the basis of the current environment."