Cox Insurance Holdings expects to lose £30m from the World Trade Centre attacks.
Cox said its reinsurance costs would rise to £12m.
Announcing its results for the six months to June 30, the company warned shareholders it would not pay an interim dividend.
Cox announced an operating profit on continuing operations of £23.9m, up from £16.3m for the same period last year.
Chief executive Michael Dawson said: “Following this attack, the market is moving swiftly to impose tight underwriting conditions and is demanding very strong rating improvements.
“We will ensure the resources and disciplines are in place to optimise future profits for shareholders.”
Most of the £30m loss from the World Trade Centre attacks relates directly to property damage near the destroyed twin towers and reinsurance.
Cox did not have exposure to the airlines, workers' compensation, life or liability losses.
Strong growth in motor underwriting helped the company's retail division make an operating profit of £17m in the period covered by the report, compared to £7m for the same six-month period in 2000.
This was partly put down to a 15% rise in premiums.