WPP first approached TNS in March, proposing to sell Kantar to TNS in return for shares, which would allow WPP to acquire the company without paying a premium for it. That proposal was also rejected.
TNS in a statement said its board determined that an acquisition by WPP, proposed May 3, would not be in the best interest of its shareholders.
"The board has no hesitation in rejecting this opportunistic proposal as it substantially undervalues the company even on a stand-alone basis" said TNS Chairman Donald Brydon.
A WPP spokesman called the rejection "cavalier." "We submitted a proposal on Saturday, and they came back on Sunday with a straightforward rejection without furnishing the [same] information that they gave to GfK," he said.
WPP Chief Executive Martin Sorrell has made it clear he wants to wants to build out WPP's research capabilities. There have been reports that WPP will now turn its attention to Nielsen Co.
"It's premature to talk about anything along those lines. We are interested in acquiring TNS," the WPP spokesman said.
Into Nielsen territory
TNS, which is well-known for measuring advertising spending, also has made strides in TV-audience measurement, an area that competitor Nielsen has long dominated. Earlier this year, TNS made a deal with DirecTV to cull data from 100,000 of the cable provider's digital set-top boxes.
TNS in a statement said a merger with Gfk would "deliver a greater range of value-added solutions more effectively to all its clients. The combination would also significantly strengthen the syndicated business of the merged group, particularly in consumer panels and media-audience measurement."