Wellington Underwriting made a massive £69.3m pre-tax loss for last year.
The result, down from a profit of £4.4m for 2000, includes a charge of £73.4m from the 11 September attacks.
Wellington has scrapped its final dividend for shareholders.
The company's share price was unchanged this morning at 46.9p.
Wellington's huge WTC loss comes because of its exposure to aviation and US property risks and also to catastrophe reinsurance.
Its Lloyd's syndicate 2020, which is managed by a subsidiary, drives much of the group's performance.
It wrote gross premiums of £868m last year and incurred net losses of £415m, with a combined ratio of 143%.
The combined ratio shows claims and costs as a percentage of premiums.
The cost of the WTC attacks was concentrated on Wellington's non-marine and reinsurance divisions, although no part of the business escaped unscathed.
The group estimated its gross cost of the terrorist attacks at £267m before reinsurance and £73m net of reinsurance.
It was also hit badly by the downturn in stockmarkets. Its return on equity last year of 45.1% produced a loss after tax of £46.1m.
Chief executive Julian Avery said: "Whilst we have suffered a substantial loss as a result of the tragic events on 11 September, this financial setback should not overshadow our strategic developments and the continued quality of Wellington¹s underwriting performance where results and forecasts remain in line with, or ahead of, expectation."
The group said current market conditions were among the hardest for many years. It was seeing rates about double their 1999 levels with particularly strong increases in aviation, energy and general non-marine business.
