Advertising giant WPP has made a £1.08 billion takeover bid for research firm Taylor Nelson Sofres (TNS).
The TNS board has so far rejected three previous bids that will see the firm merge with WPP's Kantar division.
TNS, however, is planning to continue its merger with German rival GfK instead.
The WPP offer is 52 per cent over the TNS closing price of April 28th the day before the merger plans were released.
Sir Martin Sorrell, chief executive of WPP, said: "We believe that the offer for TNS generates value for WPP share owners and offers TNS share owners both cash certainty and equity upside.
"Although our offer may be characterised by some as a hostile bid, we believe that it is in no way hostile to TNS share owners nor to TNSs clients and people. In fact, WPP believes it is more committed to maintaining the TNS brand than GfK.
The offer from WPP is a superior alternative to what is, in effect, a nil-premium reverse takeover of TNS by GfK and a merger of unequals. We remain willing, at the shortest of notice, to meet with the board of TNS.
The TNS board today stated the bid it "believed and continues to believe, [the bids] substantially undervalue the company even on a standalone basis".
It added it was committed to going ahead with the merger stating it would "deliver significant value" to shareholders.
Sir Martin said: "The offer from WPP is a superior alternative to what is, in effect, a nil-premium reverse takeover of TNS by GfK and a merger of unequals.
"We remain willing, at the shortest of notice, to meet with the board of TNS.