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.
Long-running speculation
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.
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