A new high of $50bn (£33.9bn) is being mooted on claims for Hurricane Katrina, which has caused chaos across the US Gulf Coast.
Brit chief executive Dane Douetil said that while it was too early for a specific estimate, there was no question over Katrina's significance.
"Be in no doubt this is the largest insured loss that has ever occurred."
He said total insured losses could be in the region of $50bn - well above previous estimates. Catastrophe risk specialists Risk Management Solutions predicted losses of $35bn (£23.7bn).
Eddie McLaughlin, managing director of Marsh Risk Consulting, said hurricane losses were difficult to predict accurately as not all hit land.
He said a chief lesson of Katrina was business continuity. "Were [plans in place] acted on quickly enough? It appears they may not have been tested sufficiently either."
He said oil rig construction and design in the region was also likely to be subject to investigation.
According to analysts Fitch Ratings: "Katrina will generate significant losses for the reinsurance sector and contribute to further hardening of property/catastrophe premium rates for exposures in the south-eastern portion of the US."
Munich Re has estimated it will pay out €400m (£270.9m), Hannover Re €250m (£169.3m), and Royal & SunAlliance estimated £25m of claims.