Sir Martin Sorrell will have to bid at least 300p a share - £1.2 billion - for Taylor Nelson Sofres to prevent the proposed £2.2 billion merger between the British market researcher and Germany's GfK, analysts said.

The City is bracing itself for a counter-bid from the WPP chief executive, after TNS formally unveiled its nil-premium, 50-50 merger with GfK.

WPP was last night understood to be studying the situation, leaving all its options open.

Yet sources close to the TNS-GfK deal said that it would still be possible to rebuff a higher bid from WPP by slightly altering the terms of the merger, possibly either by GfK providing a cash sweetener, or allowing TNS to have slightly more than 50 per cent of the enlarged group.

David Lowden, the chief executive of TNS, and chief executive designate of the enlarged company, said that WPP would “have to make a proposal at a premium to the current share price” to have any chance of success.

WPP had made two previous bids, the last of which is worth 237p per TNS share. Mr Lowden added: “So far WPP hasn't even valued TNS appropriately”.

TNS and GfK yesterday pledged to achieve £76 million of pre-tax synergies in the third year after the deal. Had the businesses been combined in 2007, operating profits before tax and interest for the market research group would have totalled £218.4million. Following the presentation, TNS shares last night closed at 259p, up 3p.

The combined business, which will be listed in London and called TNS-GfK will have about 24,000 staff.

Mr Lowden said the level of redundancies required would be “significantly less than a thousand” - and the actual level is estimated at nearer 500. It will cost £94 million to achieve the savings target by 2011.

Under the proposed deal, TNS will issue 11.74 shares for each GfK share. GfK's controlling shareholder, a German trade association, the Verein, which acts to further market research, will be left with 28.7 per cent of the enlarged group.

The Verein will have the right to nominate one board member as long as its stake is over 15 per cent, and the company has to retain the GfK name, maintain a base in Nuremberg, and commit to market research as “a core activity” in the future.

Mr Lowden, though, insisted that the investor would not act as a poison pill preventing takeover.

GfK was spun out of the Verein, which is run along the lines of a sports club, and the shareholder has been gradually diluted in a range of deals starting with GfK's flotation in 1999 and followed by the acquisition of NOP from United Business Media.

In a note to clients, Edward Hill Wood of Morgan Stanley estimated that a fair value for TNS, with the synergies included, was 285p.

The same note estimated that WPP could bid 300p a share for TNS and the deal would be 4 per cent earnings enhancing for WPP investors, implying that Sir Martin Sorrell could afford to bid more if he wanted to.

The two companies also abandoned plans to create a joint chairmanship, with Donald Brydon from TNS stepping aside in favour of Hajo Riesenback.

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