WPP Group, the UK advertising giant, is considering making a hostile approach for Taylor Nelson Sofres (TNS) as leading shareholders in the market research company demand that a £950 million offer must be "substantially" improved.

TNS, which is currently in talks about merging with Germany's GfK, rejected WPP’s 230p-a-share bid at the weekend, prompting expectations the advertising group led by chief executive, Sir Martin Sorrell, will take a hostile route and begin to approach shareholders directly.

However, one leading shareholder told The Times: “Sorrell is going to have to offer a lot more. We would want substantially more - at least 300p per share.”

WPP is understood to be preparing to talk to TNS shareholders once investors have a clearer idea of the cost synergies the GfK-TNS merger would produce and the value of the combined group. Forecasts range from between £30 million worth of synergies to £80 million. TNS is likely to come under pressure from shareholders in the next few days to provide a figure.

Andy Brown, chief executive of Cedar Rock Capital, TNS’ largest shareholder with a 9 per cent stake, said: “This is an industry with prospects for growth. Three leaders in the industry - WPP, TNS and GfK - think it should consolidate and there could be others. What we await now is a detailed proposal from TNS and GfK.”

WPP is seeking the same information that TNS has given to GfK as part of their merger discussions. However, it is understood that TNS is not prepared to give WPP the access it is asking for. Sources with knowledge of the deal have indicated that WPP would consider going hostile if TNS refuses to comply.

Following the merger announcement with GfK, The Times revealed last week that WPP was closely monitoring the situation and considering an approach for TNS.

Sir Martin said today that he was “surprised and disappointed” that TNS had rejected WPP's offer. He said WPP was reviewing its position and encouraged TNS shareholders to urge the board to “engage with us rather than simply persisting on an exclusive basis with a ‘nil premium merger arrangement with GfK”.

TNS announced on Sunday that it had rejected a 230p per share offer from the advertising giant.

The board of TNS said in a statement that it had "unanimously rejected" WPP's offer on the grounds that "it substantially undervalues the company even on a standalone basis".

WPP’s offer was priced at a premium of about 9 per cent to Friday’s closing price. Shares in TNS today soared more than 11 per cent to 240p .

Sir Martin added: “We are puzzled that our attempts to engage with TNS management on a friendly basis have been hindered and resisted. We are disappointed that TNS has not indicated that it would be willing to provide us with the same information which it has given to GfK as part of their nil premium merger discussions.”

The Nielsen Company, owned by Kohlberg Kravis Roberts and other private equity firms including Blackstone and Hellman & Friedman, is also understood to be mulling an approach for TNS following the GfK-TNS merger announcement. Sources said that private equity groups are also running the slide rule over TNS.

Separately, WPP’s annual report revealed yesterday that Sir Martin received a total remuneration package of £3.57 million in 2007, plus another £18.2 million of shares in his long-term bonus package. He will not collect the 2.95 million shares until 2011.

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