Elderly and "vulnerable" people with health problems and pregnant women should avoid eating raw oysters, the government food watchdog has advised after research revealed that 76% of British-grown shellfish are infected with norovirus.

Publishing the research – the first of its kind in the UK – the Food Standards Agency (FSA) urged consumers to be aware of the risks of eating fresh oysters.

The study, conducted on behalf of the FSA, found that 76% of oysters tested from UK oyster-growing beds had traces of the infectious bug, which causes symptoms such as vomiting and diarrhoea that typically last for not more than two days. The FSA said it was "difficult to assess" the potential health impact of the findings because their research was not able to distinguish between infectious and non-infectious viruses in the samples.

The research is part of continuing work by the FSA, and will feed into 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 norovirus in oysters should be.

Andrew Wadge, chief scientist at the FSA, 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."

As part of the study, scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas) took samples from 39 oyster-harvesting areas across the UK over two years. They tested 844 samples in total for norovirus DNA. Every harvesting area had at least one norovirus-positive oyster. Of the 643 samples that tested positive, 335 had low levels of the virus (less than 100 virus copies per gram). The other 308 samples had medium or high levels of the virus. Nine samples registered virus levels of over 10,000 virus copies per gram.

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, the Michelin-starred Fat Duck restaurant, owned by Heston Blumenthal, was forced to close after more than 500 people fell ill with norovirus. Raw oysters and clams were later identified in a damning report by the Health Protection Agency as the main source of the contamination.

Oysters filter large volumes of water to get their food, and any bacteria and viruses in the water can build up within them. They are subject to "purification" controls before and after harvesting which help remove these.

Demand for British-grown oysters has soared in the last couple of years, driven by issues around sustainability and ethical sourcing and after serious shortages in France. Retailers have sought to reassure consumers after the latest findings from the FSA.

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 one million people in the UK catch norovirus every year. It is the most common viral cause of diarrhoea and vomiting in the UK, according to recent agency research. It is highly infectious, but the illness is generally mild and people usually recover fully within two to three days. There are no known long-term effects.

Amid fears in the industry that sales in the British oyster market – worth £20m annually – might be affected by the new findings, the main trade body sought to reassure consumers. David Jarrad, director of the Shelfish Association of Great Britain said: "Just because we know about it [norovirus] does not mean that we should be frightened by it. The more important and meaningful figure, in our view, is that low levels of the virus were found in 52% of the positive samples."

But a group of leading chefs, headed by Raymond Blanc, were so worried by the implications of norovirus that three years ago they approached a company to find out if a norovirus -free oyster could be produced. The Cornish Shellfish Company now grows them, although the more intense purification process means consumers pay up to double the price. Dr Steve Kestin, director of the company, said: "I am not surprised by the 76% figure from the FSA. There is clearly a problem with norovirus, which is why we are producing an oyster which is safe for consumers to eat."