Unions representing Carrefour staff across Europe have criticised the demands made by one of the retailer's investors for changes to how the company is run.

The unions called proposals put forward by investment fund Knight Vinke "hostile and highly detrimental" to the world's second-largest retailer.

Last week, Carrefour issued another profit warning, prompting Knight Vinke, which owns 1.5% of the French retailer, to demand the company to appoint an independent chairman.

In an open letter to the Carrefour board, shareholders and employees, Eric Knight, the fund's CEO, said the retailer's performance had been "very poor". He questioned how the retailer was running its programme to revamp 500 hypermarkets as Carrefour Planet.

Mr Knight asked whether Carrefour chairman and CEO Lars Olofsson could judge the performance of the project, which the investor called "one of the most ambitious tranformation programmes ever undertaken in retailing".

"[Olofsson] is both supervisor and supervisee, at least as far as Planet is concerned. As chairman he is required to form a judgment on the performance of the project - either good or bad - whilst being at the same time the project's initiator and the person responsible for its implementation. This situation is far from being ideal," Mr Knight wrote.

The fund also put forward the idea of the retailer having two chief executives - one for France and one for emerging markets. The move, Mr Knight said, would reduce "complexity" at the business and increase efficiency.

However, the unions said they supported Carrefour's strategy and accused Knight Vinke of looking to "destabilise" the retailer to make it easier to break up the business.