Amlin, the biggest independent Lloyd's operator, has increased its estimate of losses from 11 September.
Its estimate of WTC losses due to Syndicate 2001 on the 2001 year of account has increased to £85.4m ($129.7m), or 14.8% of capacity at quarter end rates.
In December last year, the company quoted its estimate in sterling at £60m ($91m at today's rates) before tax, which it then confirmed on 23 January.
By 24 May the estimate was quoted in dollars and had crept up to $112m (£73.8m), or 13.3% of capacity.
The company argued that the increase of 1.5% of capacity, was offset by an increase in forecast results for Syndicate 2001, of which the company owned 69.9% of its capacity last year.
The syndicate's forecast result has improved by 2.5% since 24 May.
It is now forecast to lose between 1% and 6% of its capacity, including the WTC losses.
Excluding WTC losses, the forecast changes to a profit of between 8.8% and 13.8% of its capacity.
Syndicate 2001's gross written premium income was up by 54% in the first half of 2002 compared to the same period last year.
It received £510m in the first half, net of brokerage, compared to £330m last time.