Estimate comes as BP increases its clean-up costs to almost $40bn

The Deepwater Horizon oil rig explosion in the Gulf of Mexico could result in insured losses of as much as $6bn (£3.7bn), according to Fitch Ratings.

The explosion was “a material event for the reinsurance sector”, said Fitch managing director Chris Waterman, during a presentation in Zurich.

BP, which does not buy insurance, will cover most of the expenses linked to the clean-up, but Waterman said the disaster may cost insurers between $4bn and $6bn, with a total economic loss of $35bn.

A Moody’s report from June said insurers are now charging 50% more for policies covering oil rigs after the explosion in April triggered the worst spill in US history.

BP has also increased its estimate for cleaning up the oil spill by $7.7bn to nearly $40bn. The charge, which BP revealed in its latest results, cut the oil company's third-quarter profit to $1.8bn from the $5bn it made in the same quarter last year.

The company made a loss of $17bn for the second quarter of 2010.