Apax Partners, the private equity group, has held talks with GfK, the German market research group, about funding a cash bid for Taylor Nelson Sofres (TNS).

The news comes as analysts have expressed concern over whether GfK will find the necessary cash to trump the £1.1 billion offer for the market research consultancy already tabled by Sir Martin Sorrell’s WPP.

While GfK is still believed to be in talks with Gunter Herz, the billionaire heir to the Tchibo coffee empire, and his sister Daniela Herz, whose investment vehicle Mayfair is expected to provide some backing, a large proportion of the financing is still required.

GfK has spoken to a number of potential funding sources in the United States and Europe in the past few weeks. Some private equity parties recently mooted as interested partners, such as Cinven, KKR and Permira, are not believed to be involved.

However, The Times has learnt that Apax had held early discussions with GfK in the past week, although there is no certainty that the pair will reach an agreement. One source close to the market researcher said: “GfK is having lots of conversations with lots of people and hasn’t decided what to do yet.”

The news came as TNS told the market last night that it had posted its defence document to shareholders. The circular sets out the board’s reasons for rejecting the WPP offer, mentioning TNS’s position as a market leader, its significant investment in the internet, strong performance in 2007 and continued strength in 2008, and the fact that previous acquisitions in the sector have valued target companies substantially higher.

Donald Brydon, the TNS chairman, said that his shareholders should ignore what he called WPP’s opportunistic offer. TNS shares were up 5p, or 1.85 per cent, at 275p yesterday and the group’s market value is about £1.12 billion.

Scepticism has grown recently about whether an offer from GfK would be able to counter WPP’s latest approach. Sir Martin has offered a mix of shares and cash worth about £1.1 billion. Analysts at Panmure have raised doubts that TNS’s former merger partner would come up with the necessary cash to counter the unsolicited bid.

GfK said last week that discussions in relation to a possible offer for TNS were progressing well, but it did not provide details. It also reiterated that no assurance could be given that an offer would be made.

Times Recommends

• Minister in talks with Saudis to protect creditors
• Premier Inn hits back in £29-a-room row
• Bankers hit back at 'populist' bonus supertax