The move, urging shareholders to reject the unsolicited proposal from Sir Martin Sorrell's communications and advertising group, follows WPP's announcement on Friday that it plans to extend its initial offer period to 12 September, 2008.
WPP has offered 173p in cash and 0.1889 of a new WPP share, valuing TNS at around £1.2bn, but so far, has only received 8.61% of acceptances.
A statement by TNS said: "The board continues to recommend that TNS shareholders reject the WPP offer, by taking no action in response to the offer."
A successful merger with TNS would make WPP the world's second-largest market research company by revenue after Nielsen.
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