Royal & SunAlliance's (R&SA) losses from the World Trade Centre attacks have risen to £200m, £50m higher than its original estimate.
R&SA announced the leap in costs to £200m last week, as it continued to receive property, workers' compensation and aviation claims from the US.
The £200m figure is before tax and after reinsurance payouts.
An R&SA spokesman said the increased losses could not be attributed to any particular line of business.
"There's not one specific reason, it's just very difficult for any insurer to precisely quantify exactly how much the claims will cost," he said.
"We've got claims incoming and reinsurance contracts we need to look at further."
R&SA had been one of the first insurers to announce its losses, having made a statement three days after the September 11 terrorist attacks.
R&SA's announcement on increased losses followed similar statements by many of its competitors, some of which updated their loss estimates within a week of announcing initial figures.
The attacks are expected to cost the insurance industry up to £35bn.
The spokesman said the company would give more details on its losses when it released its third-quarter results in early November.