Two senior managers have resigned from Fusion's London office to join Mitsui. Regional underwriting manager for London, Paul Hurdle, and national broker development manager, Mark Stevenson, resigned on Friday.
A further four staff have also left the office, with two joining AIG Insurance, one going to Aspen and another leaving the industry, it is understood.
Fusion chief executive Kevin Pallett said he was "disappointed" by the departures, but he added: "When you employ good people they will be coveted by competitors."
Pallett rejected speculation that Fusion's London office would be forced to close, saying: "There are no plans to close the London office, we still have 50 people based in London."
Meanwhile, Fusion's owner, The Towergate Partnership, announced a restructure of its operations this week in a bid to reach £3bn gross written premiums by the end of 2007.
The company will be split into two divisions: broking and underwriting. Clive Nathan will head underwriting, while Amanda Blanc will head the retail broking division.
Blanc and Nathan will report to Andy Homer, group chief executive.