Ace International has issued a writ against its former group president and chief executive John Charman.
The legal action concerns an alleged breach of contract following his departure from the global insurer earlier this year. Charman left the Bermuda-based company in March. At the time, this was reported to have been because of "differences over personnel matters".
He had been promoted to his post just six weeks earlier. His departure shocked the market.
Charman received $110m (£77.1m) worth of shares when Ace bought his underwriting agency for £349m three years ago.
He has now joined Axis Specialty, Marsh & McLennan's new $1bn (£680.4m) insurance and reinsurance operation, as chief executive.
Ace spokeswoman Wendy Davis Johnson said: "This litigation involves certain restrictions and we expect Mr Charman to be bound by the terms of those restrictions."