Under the terms of the deal, TNS will offer 11.74 new shares for each GfK share and both companies will own 50 per cent of the combined business.
The new company, which will be called GfK-TNS, will be the world's second-largest market research firm after Nielsen, with offices in 111 countries. The merger is expected to create cost savings of around £76 million by the end of the third full year following the completion of the deal.