Aon could be left to untangle over £100m of claims if Equitable Life succeeds in its £3.75 billion claim against Ernst & Young.
Aon is understood to look after all the top four accountancy firms, organising up to £400m of cover through a mixture of captives and market based cover.
The remaining cover would be provided through a combination of offshore funds and self-insurance.
A market source said: "The major league accountants have no relevance or need to be in the standard insurance market."
Equitable Life is suing Ernst & Young for negligence in carrying out its auditing. Equitable chairman Vanni Treves is reported to have said he is looking for "a deep pocket anywhere that could be made to pay up".