Weinberger, the former Lowe London chairman, has been warned by IPG he will be held to his full notice period. Ed Morris, who met the IPG chairman, Michael Roth, this week, was told he faces similar restrictions if he goes.
IPG has also filed charges against Sir Frank with the American Arbitration Association in New York to stop what it claims is his use of confidential information gained while he was the head of Lowe Worldwide, as well as his poaching of Lowe executives.
Sir Frank refuted the allegations and has threatened to sue IPG for defamation.
David Hackworthy, DDB London's chief strategic officer, has confirmed he will join the start-up as a partner. However, neither Morris nor Mark Cadman, the JWT managing director, who are also slated to join, had resigned as Campaign went to press.