Taylor Nelson Sofres (TNS) fired a fresh salvo yesterday in its increasingly bitter war of words with WPP, the advertising giant that is trying to buy it.

Hard on the heels of rejecting WPP's latest £1 billion approach, TNS, the world's third-largest market research and market information provider, accused its suitor of circulating misleading comments about it.

It rejected claims that it had not provided the advertising group with all the information that WPP required and it criticised comments made by Sir Martin Sorrell, the chief executive of WPP, about where the market researcher's share price would be without the bid interest from WPP.

On Thursday, TNS, which has agreed a merger with GfK, its German industry peer, told WPP to increase its £1 billion offer after the market research group rejected a third approach at 260p a share.

TNS said yesterday that it believed that WPP had all the information required to value TNS. “WPP continues to demand that TNS open its books and treat it fairly in terms of information disclosure,” it said.

“WPP has all the financial information on TNS that has been shared with GfK. This is more than is required to value a public company ... WPP is a competitor of TNS and TNS does not intend to share information with WPP that it has not shared with GfK.”

In its statement, TNS also criticised a potential merger of Kantar, WPP's market research business, with TNS. “WPP's Kantar-TNS proposition may offer scale but it does not fundamentally change the client offering,” it said.

TNS said that WPP's claims about what the TNS share price would be without the merger talks were misleading: “WPP has stated that if TNS moved in line with the media sector, its share price would be significantly lower.

TNS is the only UK-quoted, independent market information company, operating in an industry that is demonstrating significant growth. Comparing TNS with a wider media index is misleading.”

Sir Martin was not available for comment.

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