Research published today by the Food Standards Agency shows that a significant proportion (76%) of oysters tested from UK oyster growing beds contained norovirus. The virus was detected at low levels in more than half of the positive samples (52%).

Dr Andrew Wadge, the FSA’s chief scientist, told the BBC the research has not identified any new food safety risk.

"If you are someone who enjoys eating raw oysters and you want to continue there is nothing here to say that you are at more risk or less risk. What we do say is that there is some risk."

It is difficult to assess the potential health impact of these findings, as the available research techniques are not able to differentiate between infectious and non-infectious norovirus material within the oysters. Furthermore, a safe limit for norovirus has not been established.

Between 2009 and 2011, scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas), on behalf of the Food Standards Agency, took samples from 39 oyster harvesting areas across the UK. More than 800 samples of 10 oysters each were tested.

Oysters filter large volumes of water to get their food, and any bacteria and viruses in the water can build up within the oyster. Controls before and after commercial harvesting of oysters, such as re-laying and depuration, provide good protection against harmful bacteria, but are less effective at removing viruses from live shellfish.

Re-laying is a purification process used to treat bivalve shellfish. Shellfish are harvested from a contaminated area and moved to clean areas, where they are placed on the ocean floor or into containers laid on the ocean floor, or suspended in racks. Re-laying will generally be for at least two months.

Depuration is a purification process used commercially and regulated by the Food Standards Agency. It is commonly used by producers to reduce or eliminate microbiological contamination in oysters and other shellfish. Shellfish are placed in tanks of clean re-circulating seawater, treated by UV irradiation, and allowed to purge their contaminants over several days. In the UK a minimum purification time of 42 hours is required.