Marsh & McLennan, the world's largest broker, has been hit by a credit rating downgrade from Moody's Investors Service just five days after New York State attorney general Eliot Spitzer launched a lawsuit alleging it had accepted $800m (€640m) in improper commissions.

Moody's cut Marsh's credit rating by one notch on Tuesday to low single-A and left the rating on review for a possible further downgrade "because it is not yet possible to determine the full extent of the negative effect of these developments on the firm".