More than three-quarters of British-grown oysters have been found to contain norovirus, known as the 'winter vomiting bug', experts have warned.
A study conducted by the Food Standards Agency revealed that 76% of British-grown oysters are contaminated with the highly infectious bug, which causes symptoms such as vomiting and diarrhoea.
The Government watchdog is now urging consumers, particularly those most vulnerable such as children, the elderly and pregnant women, to be aware of the risks of eating fresh oysters.
Scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas) took samples from 39 oyster harvesting areas across the UK. Low levels of the virus were found in 52% of the positive samples, according to the data.
The results will be used as part of a study being carried out by the European Food Safety Agency in order to advise the European commission on what a legal safe level for novovirus in oysters should be.
Oysters get their food intake by filtering large volumes of water, which can lead to a build up of bacteria and viruses found in the water. Infection can be passed on to consumers unless the oysters are thoroughly cooked.
Andrew Wadge, chief scientist at the Food Standards Agency, said: "This research is the first of its kind in the UK. It will be important to help improve the knowledge of the levels of norovirus found in shellfish at production sites. The results, along with data from other research, will help us work with producers to find ways to reduce the levels of norovirus in shellfish, and work within Europe to establish safe levels."
He went on: "Although oysters are traditionally eaten raw, people should be aware of the risks involved in eating them in this way. The agency advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish to reduce their risk of getting food poisoning."
David Lees, the lead investigator at Cefas, said: '"Norovirus is a recognised problem for the sector, and this study provides important baseline data to help the industry and regulators to focus on the key risks."
In 2009, Heston Bleumenthal's Michelin-starred restaurant, Fat Duck, was forced to close after more than 500 people fell ill with norovirus. Raw oysters and clams were later reported by the Health Protection Agency as the main source of the contamination.
Waitrose said in a statement: "Food safety is our absolute priority and we're in full support of this research. All our oysters are from a co-operative of known farms in Scotland who follow strict industry guidelines, and it is important to note that the FSA research looks at oysters from oyster beds, not those on sale ready for eating. Government advice hasn't changed, and we would support the recommendation that the young, elderly, unwell and pregnant should avoid eating raw or lightly cooked shellfish."
Between 600,000 and 1m people catch the norovirus in the UK every year.