Sir Martin Sorrell, chief executive of WPP Group, and Richard Desmond, owner of the Express newspapers, have traded lawsuits over media buying work done by the advertising group for the US launch of Mr Desmond?s OK! magazine.

Northern & Shell North America, the publisher of OK! in the US, said it had lodged a claim on Friday with the High Court in London, alleging breach of contract and seeking damages of at least $5.5m from Mediacom Worldwide, WPP?s media buying subsidiary.

WPP said it had filed two complaints with a New York federal court the previous day, seeking $10m for ?non-payment of media bills? relating to OK!?s launch last August.

Ian Rosenblatt, a lawyer for Mr Desmond, yesterday claimed Mediacom, a subsidiary of WPP, had encouraged Northern & Shell to spend more than the $3m on media buying it had budgeted for the US launch, in return for an agreement that it would find buyers for 15 pages of advertising a week in the title.

The expectation of that advertising led it to increase the number of pages in the first issue of OK!, at a cost of at least $1.5m, Northern & Shell claims.

A spokesman for WPP denied that its media planning and buying division had offered any such guarantees.

Northern & Shell?s claim highlights the increasingly common practice of clients seeking guarantees from media buyers in return for placing business with them.

The dispute comes as WPP is involved in an escalating legal row with Marco Benatti, its former country manager in Italy.

WPP is investigating evidence that suggests Mr Benatti had a hidden interest in a company that WPP bought.

WPP has taken the first steps in filing a suit against Mr Benatti, who says he has acted properly and has filed a wrongful termination suit against WPP.
Additional reporting by Gary Silverman