The Takeover Panel made the ruling following a request by Aegis Group for a deadline to be drawn for the long-winded process. Robert Lerwill, chief executive of Aegis, has also said in the press that he wants WPP to make a decision "sooner and not later".
WPP and Hellman & Friedman first showed interested in Aegis in September, after rival advertising group Publicis admitted it was looking at a possible bid.
Publicis has since withdrawn from the race, leaving WPP and Hellman & Friedman as the only official contenders for Aegis.
WPP is interested in the acquisition because Aegis owns the market research network Synovate. If it did buy Aegis, along with Hellman & Friedman, it is likely the company would be broken up and media agencies Carat and Vizeum, as well as digital network Isobar, would be sold off.
In a statement today, the Takeover Panel said: "Following discussions with the parties' advisers, the panel executive has ruled that WPP and Hellman & Friedman must, by 12 noon on November 25 2005, either announce a firm intention to make an offer for Aegis under Rule 2.5 of the code or announce that they do not intend to make an offer for Aegis."
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