Ratings agency Fitch said Lloyd's could face gross liabilities of up to £7bn from the terrorist attacks on the United States, according to a report in the Financial Times.
Last week, the insurance market estimated its net exposure, which includes collected reinsurance, at £1.3bn.
Fitch said Lloyd's net loss was likely to be at least £1.7bn because history had shown initial estimates of the cost of large catastrophes were always understated, the paper said.
After consulting clients, Fitch reckoned gross exposure tended to be three to four times the net figure.
On Monday Lloyd's told the FT its £1.3bn net forecast could be revised upwards but it was confident it would not have to be increased substantially.
