Market research group Taylor Nelson Sofres today opened its books to rival WPP despite agreeing a £1.5bn (€1.9bn) tie-up with German firm GfK last week.
TNS said WPP had requested to see its books as the persistent suitor refuses to step down following news of the merger.
WPP’s move suggests the FTSE 100 Index advertising and communications giant could be preparing for a takeover tussle over TNS.
WPP has already made two unsolicited takeover approaches, which have been rejected by TNS for “substantially” undervaluing the company.
TNS has not previously disclosed confidential information to WPP due to its rival status.
However, it will hand over the information after agreeing confidentiality restrictions as stipulated under Takeover Code rules.