Sir Martin Sorrell's WPP raised the stakes in the £1 billion battle for market researcher Taylor Nelson Sofres today by forcing it to reveal key financial information.

TNS is in the throes of a £2 billion friendly merger with its German rival GfK and has so far rejected two takeover approaches from WPP, the last of which valued it at 236p a share. Today its shares were comfortably above that level at 259p.

WPP has demanded the new information from TNS under Takeover Panel rules that say any serious bidder must receive "equality of information". This means that WPP will now sign a confidentiality agreement and be given access to the same detailed information as the Germans.

Analysts have been split on whether the merger with GfK, which because it is a nil-premium, all-share-deal offers no uplift for shareholders, will be as attractive as a hostile bid from Sorrell. So far, he has suggested that any offer will be a mixture of cash and shares. Analysts' forecasts for a successful take-out price range from as low as 240p to as much as 300p.

TNS today said that it "continues to believe [the WPP offers] substantially undervalue the company even on a standalone basis". It added that it also continued to believe that "a combination of TNS and GfK will deliver significant value to TNS shareholders through accelerating revenue opportunities and substantial cost savings".

But Sorrell does not believe that a merger of equals works, comparing it with "two drunkards propping up a lamp-post between them".

He says he is not desperate to clinch the deal. But analysts point out that if he pulled it off, WPP would leapfrog Omnicomagain to become the largest advertising group in the world.