Reports of a $10.5bn takeover bid of Aon from Willis have been denied by both parties.
There have been rumours that the rapidly expanding Willis was seriously considering making a move on Aon.
If the merger were to go ahead it would create a group which could easily challenge Marsh with a set-up value of around $16bn.
Reports claim the new group would almost certainly be headed by Willis chairman Joe Plumeri, who has lead the company through a successive amount of takeovers resulting in massive profits.
Aon had grown into a group largely regarded as too fragmented, reports said.
As a result its performance has weakened and Willis executives are thought to be intrigued at the prospect of a takeover.
