Lloyd's of London says it is �325m out of pocket over the policy

The dispute centres on a policy Lloyd's bought through Aon and Benfield to protect its central fund of assets.

Last year Lloyd's was left out of pocket when it lost a legal battle to force six insurance providers to pay out �500m over the disputed policy.

It got only �152m and now wants Aon and Benfield to pay the �325m difference. Both Benfield and Aon said they were contest the action.

'Vigorous defence'

A Lloyd's spokeswoman said on Friday that it had informed Aon and Benfield of its intention to sue them.

The case is expected to take up to 18 months to come to court.

Benfield said it was "disappointed that Lloyd's has considered it necessary to take this action".

"We continue to believe, based on professional advice we have taken, that these proceedings will not result in a material liability to Benfield," Benfield added.

"Obviously, Benfield will defend any action vigorously."

Aon said it was also "disappointed" that Lloyd's had chosen to pursue the action.

"We do not accept that Lloyd's has a valid claim against Aon and will be contesting it vigorously," said an Aon spokesman.