In a market announcement, the board, which has been advised by Deutsche Bank, JPMorgan Cazenove and Moelis & Company, said it recommends that shareholders accept the WPP offer, as the directors intend to do so.
If WPP wins 75% of acceptances it can delist TNS from the stock market, which means TNS shareholders who do not accept the offer could end up owning shares in an unlisted company.
TNS's statement said: "The board continues to believe that the WPP offer undervalues TNS.
"However, the board recognises that given that WPP has reduced the acceptance condition to 75% of the TNS ordinary shares to which the WPP offer relates and stated its intention to de-list TNS when it has declared the offer unconditional, there is a risk that TNS shareholders who do not accept the WPP offer could as a result own a minority interest in an unlisted company."
WPP, led by Sir Martin Sorrell, announced last Friday that it had received acceptances representing 60.86% of TNS shares. Earlier in the week it had received 43% acceptance and warned that its £1.1bn bid was final.
The group's offer will remain open for acceptance until 3pm on Wednesday October 8.