WPP is reported to be certain to make a bid for Grey, offering between £600m and £700m.
Havas will also make a bid, but since it lost its private equity partner Quadrangle it has yet to find a new one.
For Havas, a successful bid for Grey is being seen as a make-or-break move for the mid-sized French advertising company. It desperately needs Grey's US clout to balance its own weak American operations.
There is also concern from Havas shareholders about whether it can afford a bid for Grey, after the painful restructuring process it went through, which saw more than 1,500 lose their jobs.
Havas chief executive Alain de Pouzilhac has said it can afford a bid, but it is under pressure from investors not to make an offer. Financier Vincent Bollore recently raised his stake in Havas from 5.1% to 8% and is believed to be against a bid.
Only one US private equity group, Hellman & Friedman, is being tipped to submit a bid, with Providence Equity Partners and The Blackstone Group deciding to walk away from making a play for Grey.
Hellman & Friedman is being advised by the former Young & Rubicam chairman Mike Dolan.
With three companies vying for Grey's assets, which include the Grey advertising network and media buying agency MediaCom, the price could rise as high as £750m.
From the start of the process, Grey chairman and chief executive Ed Meyer has said that he is only interested in bids for the whole company and does not want to see the network broken up and sold off piece by piece.
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