Aviva said the June and July floods in the UK, the worst to hit the country in 60 years, could cost it 340 million pounds ($691 million) and would affect its general insurance results.

Britain's biggest insurer said it expected a bill of 165 million pounds from the July floods, which badly hit central and western parts of England, on top of expected claims of 175 million pounds from the June floods.

It said there was now "a degree of uncertainty" whether it would meet its target of a combined ratio of 98 percent.

The ratio is a key non-life performance benchmark that measures claims and costs as a percentage of claims. A figure below 100 denotes a profit on underwriting.

Aviva said it had exercised an option to use reinsurance to limit the maximum cost of further major claims in the UK to 110 million pounds, the cost of which is included in the 165 million figure.

The total cost to the insurance industry of this summer's flooding has been estimated at as much as 3.3 billion pounds by leading catastrophe-exposure modelling firm Risk Management Solutions.