LONDON—Three-quarters of British-grown oysters contain norovirus, a bug which causes diarrhea and vomiting, according to new research published on Tuesday by the Food Standards Agency (FSA).

But the government health experts warned that their tests did not reveal the extent of infectious norovirus, the strain that makes people ill.

Nor could they say how effective various treatment processes before and after harvesting were in getting rid of the virus, explaining that their study will contribute to a Europe-wide review of the problem.

The two-year study looking at samples from 39 oyster harvesting areas across Britain found 76 percent off oysters straight off the bed contained norovirus.

“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,” the FSA said in a statement.

“Furthermore, a safe limit for norovirus has not been established.”

In 2009, the Michelin-starred Fat Duck restaurant of top British chef Heston Blumenthal was forced to close after more than 500 people fell ill with norovirus. Raw oysters and clams were later identified as the main source.

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 controls before and after harvesting which help remove these.

Norovirus is the most common viral cause of diarrhea and vomiting in Britain, but is generally mild and most people recover within a few days.