Amlin has entered a reinsurance deal to boost its capacity for 2002 and 2003.
The £50m quota share arrangement with Montpelier Re will help push the capacity of Amlin's Lloyd's Syndicate 2001 up to £900m for the two years.
The deal is in addition to the £50m agreement with XL Re announced in March.
But the company predicted it could need a further £200m of capacity for 2003.
It forecast that Syndicate 2001 would make a 2%-7% loss in capacity for the 2001 year of account.
Excluding 11 September losses, which are estimated at 13.3% of capacity or $112m (£77m), the syndicate's result would be a profit of 6.3% to 11.3% of capacity.
The company said in a statement there was still "considerable uncertainty" over the 11 September losses and its forecasts were being made earlier than usual.
It also revised its forecast for Syndicate 1141's result for the 2000 year of account, when it stopped trading.
The forecast deteriorated to a loss of between £26m and £31m from a previously forecast loss of between £23m to £28m.