More than three-quarters of British-grown oysters contain norovirus, new research has found.
The study, conducted on behalf of the Food Standards Agency (FSA), discovered that 76% of oysters tested from UK oyster growing beds had traces of the infectious bug.
Low levels of the virus, which causes symptoms such as vomiting and diarrhoea, were found in 52% of the positive samples, according to the data.
The FSA said it was "difficult" to assess the potential health impact of the findings, as researchers were unable to differentiate between infectious and non-infectious norovirus material in the shellfish.
However, it said the results of the study would be used as part of a review by the European Food Safety Authority, which is to advise the European Commission on what a legal safe level for norovirus in oysters should be.
Currently a safe limit for the highly infectious virus, commonly know as the "winter vomiting bug", has not been established.
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."
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.
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."
Between 600,000 and one million people in the UK catch norovirus every year.