New York’s Attorney-General has subpoenaed Marsh, Aon and Willis to assist in an investigation of the way they are paid commissions by insurers. Attorney-General Eliot Spitzer says the agreements may constitute a conflict of interest.

The move is the latest in a series of intermittent protests against the three mega-brokers, which some clients allege are winning from both sides of the insurance purchase through fees from them and commissions from the insurers.

Media reports said the investigation was sparked by complaints from several sources, including the influential Washington Legal Foundation.

The three companies have promised to co-operate with the investigation, with an Aon spokesman adding the agreements are a “long-standing and common practice” in the industry, and are fully disclosed to clients.