WPP, the advertising group headed by Sir Martin Sorrell, has said its £1.2bn offer for Taylor Nelson Sofres, the market researcher, is final and will not be increased unless a rival bidder emerges.

The cash-and-share offer values TNS at 259.2p based on Wednesday’s closing price and includes 173p a share in cash and 0.1889 of a new WPP share for every TNS share.