The UK's Takeover Panel said WPP, which has partnered US private equity firm Hellman & Friedman in its early negotiations with Aegis, would have to make a formal approach or similarly announce it is ruling out a bid by the date in four week's time.
The deadline will put pressure on both WPP and Vincent Bolloré, the chairman of the Havas network, who has amassed over a 20% share in Aegis since the summer.
WPP has stressed its interest in Aegis is concentrated on the company's Synovate research wing.
A move for the entire business, including the Carat and Vizeum media buying agencies, would likely fire the attention of competition watchdogs in the UK and Europe.
The Takeover Panel said failure to observe to its directive will effectively rule out either WPP or H&R for six months in any move for Aegis.