Aon has had its senior debt rating and commercial paper rating downgraded from stable to negative by Moody's.
A series of US firms have been downgraded in the wake of the New York attorney general's probe into the use of contingent commissions at Marsh & McLennan.
Moody's said rating change was prompted by potential uncertainties associated with the investigation as well as the possibility that Aon could decide to discontinue contingent commission arrangements.
