supermarket group Sainsbury's said that a technical glitch had left shelves bare at some of its busiest stores since the start of the new year.

The group said a problem with product demand forecasting systems had caused shortages of fruit, vegetables and other fresh foods at several Sainsbury's Local convenience outlets in central London since they re-opened earlier this week.

The retailer was forced to post notices on near-empty shelves at the shops apologising to customers for the poor availability of all fresh food lines.

A Sainsbury's spokeswoman said the problem occurred when a product demand forecasting system was shut down while the central London shops were closed during the Christmas holiday.

The stores ran down their levels of fresh products over Christmas and New Year and the forecasting system failed to supply enough products when the shops were due to re-stock on Tuesday.

Sainsbury's has slipped to third place in the ranks of the UK supermarket industry, behind Tesco and Asda, due to the distribution problems and competition from rivals, who have moved into lucrative non-food areas such as clothes and financial services.

It is now battling with Safeway owner Morrisons to keep its position.

The group slumped to its first deficit in November, posting losses of £39m in the six months to September 30 against profits of £323m a year earlier.