The deadline for shareholders to accept the deal expires at 3pm tomorrow, and WPP has piled on the pressure by announcing this morning its offer is final.
WPP has not provided a fresh update of how many TNS shareholders have accepted the deal. On Tuesday it revealed acceptances had climbed from 34% of TNS shares on Friday to 43% on Monday.
TNS, which has repeatedly told its shareholders not to accept the offer, has been silent since Friday and had no comment to make this morning a sign that TNS chief executive David Lowden is feeling the pressure.
It appears to be the endgame for the UK's largest market research company, which back at the beginning of June was set on a merger with Germany's own research giant GfK that was aimed at creating a global company capable of rivalling big guns such as AC Nielsen, Ipsos, Synovate and WPP's Kantar.
WPP gatecrashed the nil-premium merger in July with a more attractive offer of 173p in cash and 0.1889 of a new WPP share per TNS share. GfK then dropped out after failing to secure the funds it needed to pursue its own bid for TNS.
Since July WPP chief executive Sir Martin Sorrell has left the offer unchanged despite the efforts of TNS chairman Donald Brydon, who met Sorrell three weeks ago to try to persuade him to make a more generous offer that could be backed by the TNS board.
WPP's offer values TNS at £1.14bn or 259.2p per share, based on last night's closing price for WPP's shares of 456.5p.
This morning TNS shares were trading at 246.8p, up 0.7% since last night, implying the market believes there is slim chance of a better deal coming along.