Kenny Maciver, managing director of Groupama, has resigned, taking the industry by surprise.
He was managing director of Lombard General Insurance before it was merged with Gan, Groupama's UK subsidiary, in May.
A company statement read: "With the integration of Groupama's operations in the UK almost complete, Kenneth Maciver has resigned to pursue other opportunities.
"His contribution during the integration process was extremely valuable."
Groupama has no plans to appoint a successor, and his duties will now be shared by the executive of the board.
Director of personal insurances Tim Ablett will be responsible for affinity and accident and health business.
Chairman and chief executive Tony Lancaster will handle finance and IT.