Knight Vinke, the US activist asset manager, has attacked Gérard Mestrallet, chairman of Suez Group, accusing him of failing basic international corporate governance standards, in the latest salvo of its fight to break up the French utilities group.

In an open letter to Suez published today, Eric Knight, managing director of Knight Vinke, which is backed and partly-owned by Calpers, the California Public Employees' Retirement System, said he was "shocked" at the lack of separation of the chairman and chief executive roles between Suez and its Belgian subsidiary Electrabel.