Fruit and vegetables being sold in British supermarkets have been found to contain norovirus, food safety experts have announced.

The UK's Food Standards Agency (FSA) researchers discovered that one lettuce in every 20 contained the vomiting virus.

The bug, spread by human waste, was also detected in one in every 27 bags of frozen raspberries.

"Norovirus is the commonest cause of gastrointestinal disease in the UK causing millions of cases annually," said the researchers, who added that their findings indicated serious negligence by food suppliers and retailers.

"One should not expect to find a norovirus in one's lettuce or raspberry," scientists told The Sunday Times.

"When the virus is detected it is clear that guidance [on food hygiene] is not being adhered to and that in consequence the pathogen is entering the food supply chain."

The researchers tested 568 lettuces, which had been mostly grown in Britain, and found norovirus in 30 of them.

Seven out of 310 batches of fresh raspberries and 10 out of 274 samples of frozen raspberries also tested positive for the bug.

Sian Thomas of the Fresh Produce Consortium, which represents salad growers, said: "The fresh produce industry is committed to maintain high standards."

The FSA said the research was part of a wider study into the sources and spread of norovirus. It has already pinpointed shellfish, especially oysters, as a key risk because they pick up the virus from human sewage pumped into the sea.