Leading shareholders in Taylor Nelson Sofres (TNS) called on WPP to increase its £1 billion offer after the market research group rejected a third approach at 260p per share.

One leading shareholder, who refused to be named, said that it would like to see WPP table an offer closer to 300p, although it may consider an offer at about 280p.

WPP wants to merge TNS with Kantar, its own market research unit, to become the second-largest market research group after Nielsen. The combined group would have estimated combined revenues of $4 billion (£2 billion), doubling Kantar's $1.9 billion figure for 2007.

However, Donald Brydon, the chairman of TNS, which has agreed a deal in principle with GfK, a German rival, said: “It is clear that WPP is determined to try and frustrate the GfK-TNS merger for the benefit of WPP's underperforming Kantar division. It is time for Sir Martin Sorrell and WPP to stop interfering and make their intentions clear.”

TNS swiftly rejected WPP's latest proposal on Wednesday night, which was made up of two thirds cash and one third WPP shares. This followed an announcement from the Takeover Panel after the market closed on Wednesday, telling WPP to make a firm offer or withdraw by July 9.

Mr Brydon said: “I think Sir Martin thinks the merger between GfK and TNS is a huge threat to his business.” He added that he believed that TNS shareholders were wondering what WPP's real intentions are. “Is Sir Martin serious or does he just was to screw up your deal?” he said.

However, Sir Martin insisted that he had the support of TNS shareholders. “Every shareholder says that we [WPP] should be given access to more information,” he said. It is understood that Sir Martin is now considering a hostile approach for TNS.

Some analysts said that the WPP offer represented a fair price for shareholders and a premium of 52 per cent on TNS's undisturbed share price of 171p.

Collins Stewart said in a note that the approach represented an “excellent deal for TNS shareholders”, whose share price has been propped up by the possibility of a WPP offer.

News of the latest approach boosted TNS's share price 18p, or 8per cent, to 243p. Analysts at Citigroup said that they expect WPP to put the offer directly to shareholders.

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