More than three-quarters of British-grown oysters contain norovirus, research has found.

The study, for the Food Standards Agency (FSA), discovered that 76 per cent of oysters tested from UK oyster growing beds had traces of the infectious bug.

Although the virus was detected at only low levels in 52 per cent of the positive samples, a safe limit for norovirus has not been established. The virus can cause vomiting and diarrhoea.

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.

Scientists from the Centre for Environment, Fisheries and Aquaculture Science took samples from 39 oyster harvesting areas. David Lees, the lead investigator, 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."