The gloves came off in the battle for market research giant Taylor Nelson Sofres today as would-be bidder Sir Martin Sorrell accused it of a"Yah boo sucks" response to his third and increased offer last night.

TNS, which is determined to go ahead with its own £2 billion merger with German rival GfK, rejected the offer from Sorrell's advertising giant WPP, which was raised from the equivalent of 220p a share to 260p, valuing TNS at £1.1 billion.

Donald Brydon, chairman of TNS, riposted: "I have huge admiration for Sir Martin but the only reason he is coming up with these continuous weak bids is that he can see what a threat our merger is to his own business."

Sorrell has now made three offers for TNS, and while today's is worth 260p, which helped push the shares up 1/4p to 2251/4p, Brydon declared: "That is nowhere near the value of our company, never mind paying any kind of premium for control."

"He's wrong," responded Sorrell. "Brydon hoisted a white flag above the company and got himself into a position where he had no option than the merger of unequals with the Germans. Without us, his share price would be languishing at 150p and he would have nowhere to go."

The Takeover Panel set WPP a "put up or shut up" deadline of 9 July yesterday.

"At last we are going to get some clarity for our shareholders," said Brydon. " So far we have been assaulted by innuendo and gossip while we've been trying to put together the merger.

"We have been endlessly patient. But now it's time for Sir Martin and WPP to stop interfering and make their intentions clear."

Sorrell responded that he would make his intentions clear directly to TNS shareholders.

"We have tried to engage with Brydon and the board and all I can say is that their approach has been inconsistent," he said.

"The arrangements they have with the Germans seem to impose some burdens on their ability to talk to us. That puzzles us. We have been willing to talk since 27 March. Now we will go straight to their shareholders and ask 'do they believe the TNS management' or do they want our offer?

"Brydon fails to deal with the basic issue. Why, if TNS is doing so well, did he get involved in this merger of unequals?"

Sorrell also dismissed Brydon's claim that WPP's own market research business was "underperforming", saying it was faring much better than TNS. He is also sceptical about TNS's assertion that the merger will produce £76 million of synergy savings.

The two sides also dispute whether or not WPP has had the same access to information as GfK.

Brydon says it has had "materially the same information" but Sorrell says he has been "denied access" to some of the information given to GfK's largest shareholder, the Verein trust.