Late last week, Interpublic issued a statement saying that Sir Frank, who was once the chairman of its Lowe Worldwide advertising network, had confidential and proprietary information relating to clients and staff and used these to set up his new agency.
Interpublic said it was pursuing its claim with the American Arbitration Association, which specialises in resolving management-labour disputes, and was seeking monetary compensation and injunctive action.
The escalation in the war of words came as David Hackworthy, chief strategic officer of DDB London, has been confirmed as resigning today to join Sir Frank's new agency.
This afternoon, Sir Frank responded with a statement calling for Interpublic to back down.
"My legal team is seeking an immediate withdrawal of the claim submitted by Interpublic to the American Arbitration Association on January 13 and an unreserved apology for the allegations contained within it and their press release of the same date," the statement read.
"The allegations are without foundation and are utterly refuted. If Interpublic does not withdraw its baseless claims we may seek damages for defamation and tortious interference."
Sir Frank's ties with Interpublic ended in March 2005, when his contract came to an end.
Since then he has been linked with speculation about a possible management buyout at Lowe Worldwide, before revealing he was starting his own agency in December last year. He was promptly joined by Lowe London chairman Paul Weinberger, who brought the 拢45m Tesco account with him.
Lowe executive creative director Ed Morris is also said to be on the verge of joining the start-up, along with JWT managing director Mark Cadman.
Paul Hammersley has resigned as chairman and chief executive of DDB London to join Sir Frank.
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