Advertising group Aegis has received a £1.57bn ($2.8bn) takeover approach, but has not yet revealed the name of the interested suitor.
Shares in London-based Aegis jumped 20% to near five-year highs on the news.
Analysts have speculated about the identity of the likely bidder, with names being suggested including rivals US-based Omnicom and France's Publicis.
Others think fellow UK-based advertising firm WPP may be the suitor, or another French company, Havas.
Aegis said the current approach remained "preliminary" at this stage.
It added that there "can be no certainty that a [formal] offer will be made".
Aegis helps companies buy advertising space and plan their marketing strategies.
It has been the subject of persistent takeover speculation for some time.
France's Les Echos newspaper has reported that Publicis was close to making a friendly takeover bid for Aegis.