Fitch Ratings has warned for the third time in six months that it could downgrade Marsh & McLennan after New York State attorney general Eliot Spitzer sued the world's largest broker accusing it of accepting $800m (€646m) in improper commissions.

The suit, which was filed by Spitzer on Thursday at the State Supreme Court in Manhattan, alleged that Marsh & McLennan received special payments from insurance companies that amounted to rewards for business that the company and its independent brokers steered to them.