WPP's complaints are in response to TNS's announcement to the media yesterday that it was opening its books to the company, which could lead to a third bid from Sir Martin Sorrell's WPP Group in excess of £1bn.
However, WPP say this is not the case. The marketing services giant says that GfK has received preferential treatment despite the fact that the German firm could initiate a takeover, rather than a merger.
GfK's largest shareholder, GfK-Nurnberg e.V, would be the biggest shareholder in a merged TNS-GfK company, with a holding of 28.7%.
In a statement, WPP said: "TNS has stated that it will be providing WPP with certain information that was provided to GfK-Nurnberg e.V. GfK's controlling shareholder.
"This is on the basis that GfK Verein is treated as a potential offeror for TNS because it could end up in control of TNS (for the purposes of the City Code on Takeovers and Mergers) following completion of its proposed merger with GfK.
"TNS has not stated that it will provide WPP with the same information that it has provided to GfK.
"WPP understands this is on the basis that, notwithstanding it is expressly stated in the announcement by TNS on June 3 2008 that GfK is free to work on a possible offer in respect of TNS and is entitled to use the information provided to it by TNS for this purpose, GfK is not in fact a potential offeror for TNS."
WPP added that it is a bona fide potential offeror for TNS and, in the interests of TNS shareholders, the TNS board should accept that all the information already provided by TNS to GfK should be given equally and promptly to WPP.
TNS shareholders and GfK shareholders will each hold approximately 50% of the share capital of TNS following completion of the proposed merger.
The exchange of words between WPP and TNS follows an attack by Sorrell on the potential TNS-GfK merger, after it emerged that the deal could face an EU probe. He accused the companies of having not "done their market research".
Sorrell also reiterated his claim that a merger between TNS and GfK would result in job losses: "As a result of this it makes the synergy and margin targets seem less realisable.
"More jobs will have to go in the UK, France and Germany if they want to achieve them.
"A premium bid in markets like these has tremendous appeal, more than waiting for TNS management to deliver synergies in three years time."