Negotiations between WPP's media buying division Group M and broadcaster Channel 4 have come to a standstill as Sir Martin Sorrell's advertising group seek an improvement on their two-year, £200m deal that expires before 2013.

According to , Media, Telecoms and Technology Editor in the Daily Telegraph, the parties are struggling to agree terms.

"Although such negotiations often become fraught, tensions are understood to be running higher than usual and the issue has been referred to the Channel 4 board. More than a quarter of the broadcaster’s advertising revenues come through Group M, handing the media buying agency enormous leverage."

Group M, with clients including Barclay's and BSkyB on their books, accounts for about 27% of Channel 4’s advertising take. “It’s a case of who blinks first,” said one source in The Independent, though also attributed to a unidentified source was the quote: “negotiations can be tense but the discussions are active. Two weeks is a long time.”

A Channel 4 spokesperson would only confirm it is “in the midst of trading season negotiations with media agencies for 2013”, whilst Group M have declined to comment.