London--Tempus Group plc confirmed Thursday that it is in talks with WPP Group plc, the advertising empire run by Sir Martin Sorrell, about a counter bid that could scuttle Tempus' agreement to be taken over by Havas Advertising. The move could possibly spark a full-scale bidding war over Tempus. Havas reacted angrily to a report in Wednesday's Financial Times about the negotiations and asked the so-called Takeover Panel to force WPP to declare its intentions. The panel declined to comment, and WPP apparently will continue to pursue a takeover bid. Tempus is already 22% owned by WPP. The discussions between Tempus and WPP hinge on the price of any WPP offer and the roles Tempus' directors would play in a WPP-owned operation. Havas is now offering $780 a share for Tempus, valuing the company at $603 million. But if WPP makes a higher cash offer, Tempus' board may be obliged to recommend it. WPP owns some of the largest advertising companies in the U.S., including Burson-Marsteller, Hill & Knowlton and Ogilvy & Mather Worldwide. Havas owns New York-based Euro RSCG Worldwide and Arnold Worldwide Partners in Boston.