WPP made its £1.078bn offer last night, on the same day that the Takeover Panel gave the company a put-up-or-shut-up date of July 9 to make a firm offer for TNS. It is up from its second offer of £996m.
TNS has already agreed a nil-premium merger with GfK, and said that it was rejecting WPP's offer because it believes it still "substantially undervalues" the company.
Statements issued by the two companies relating to the deal indicate that the battle is becoming increasingly bitter. TNS said that WPP's "successive approaches and considerable press commentary and innuendo collectively represent an attempt by WPP purely to frustrate the merger of GfK and TNS".
Donald Brydon, chairman of TNS 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."
In turn, WPP complained that it had been denied access to additional information which TNS has shared with GfK, which has meant that WPP has not been able to operate on equal terms to the potential detriment of TNS share holders.
However, TNS denied this, with Brydon saying: "WPP have received materially the same information as has been provided to GfK. WPP have got all the information they need and have had all the time they need.
"It is clear that WPP are determined to try and frustrate the GfK-TNS merger for the benefit of WPP's underperforming Kantar division. It is time for Sir Martin Sorrell and WPP to stop interfering and make their intentions clear."
Shares in TNS were up 2.4% this morning to 230.5p.