The already bitter battle for control of Taylor Nelson Sofres, the market research group, heated up today as TNS said it had rejected a second raised £1 billion cash and share offer from Sir Martin Sorrell's advertising agency WPP as too low and accused him of simply wanting to stop its agreed merger with German rival GfK.

WPP has until next Wednesday to make a formal bid or walk away.

TNS said it had received the raised £1.078 billion or 260p a share offer last night but that its board had unanimously rejected it.

WPP wants to merge TNS with its own market research unit Kantar, but TNS chairman Donald Brydon said: "It is clear that WPP are determined to try and frustrate the GfK-TNS merger for the benefit of WPP’s underperforming Kantar division."

He said the offer "again substantially undervalues TNS even on a standalone basis. The Board believes 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.

"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.

"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 time for Sir Martin Sorrell and WPP to stop interfering and make their intentions clear.’

Sir Martin said that the GfK nil-premium merger would not provide the benefits to shareholders that the boards are claiming, since two head offices - in Germany and the UK - would duplicate costs while the German side would resist any redundancies.

The Takeover Panel has given him until next Wednesday (July 9) to make a formal offer or walk away. TNS shareholders will get to vote on the merger at a general meeting on July 18.

WPP first offered 230p after GfK and TNS revealed plans for a merger in April. After this was rebuffed it raised it to 241.5p before last night's offer.

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