The market research company has decided to give WPP the same information that was provided to GfK as part of their merger agreement, after it received a notice from WPP requesting information.
said it had on the basis that it believes the offers undervalue the company.
Sorrell has said that WPP is considering raising its unsolicited offer or making a hostile bid for TNS. TNS has not previously disclosed business information to WPP, because it is a competitor.
The chain of recent events was triggered on June 3, when that will form the world's second-biggest market research company, challenging the Nielsen Company.
Separately, it emerged over the weekend that a merger between TNS and GfK could face an EU competition probe, with concerns centring on a concentration of power in the market for TV-audience measurement, one of TNS's most valuable contracts.
It is understood that the proposed merger could require behavioural remedies or else disposals to gain approval from the EU Commission.
The revelation is a surprise, as management on both sides have previously played down the regulatory worries as they trumpeted the merits of a tie-up. It is believed a takeover by WPP would also face competition issues.
News of a possible regulatory probe provoked a stinging attack by Sorrell, who told The Sunday Telegraph that the companies "didn't seem to have done their market research".
Sorrell also reiterated his claim that a merger between TNS and GfK would result in job losses.
He said: "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."
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