TNS told WPP and its chief executive Sir Martin Sorrell to “stop interfering” after snubbing the latest approach, which values TNS at £1.08bn.
TNS opened its books to WPP last month, but also criticised the firm’s “press commentary and innuendo” and accused Sir Martin of attempting to frustrate the merger with GfK,.
TNS chairman Donald Brydon said: “The board of TNS has been extraordinarily patient with Sir Martin Sorrell. We have now received three proposals from WPP, each of which substantially undervalues the company.”
And he added: “It is time for Sir Martin Sorrell and WPP to stop interfering and make their intentions clear.”