Fitch has downgraded the insurer financial strength (IFS) rating of Lloyd's of London to A minus, from A plus, and placed it on Rating Watch Negative.
The international rating agency said it had moved because Lloyd's 2001 results would not be good enough to justify the A plus rating.
Fitch said that, apart from the terrorist attacks in the US, Lloyd's already faced significant losses from Hurricane Allison, the Petrobas oil rig explosion and the attack by the Tamil Tigers on Sri Lankan Airlines.
It added the uncertainty surrounding the losses sustained in the US and poor earnings performances in recent years meant that Lloyd's ability to attract and retain capital was under threat.
Fitch said the new A minus rating had been placed on Rating Watch Negative to reflect its concerns that "losses from the terrorist attack may ultimately prove to be more severe than Fitch's current view would indicate."
The agency also opened up the possibility of further downgrades for Lloyd's rating by pointing to its business mix which includes heavy concentrations in aviation risk, liability, property and various high level catastrophic coverages.
