It was the oysters. Health officials today blamed sewage-infected shellfish for striking down hundreds of diners at Heston Blumenthal’s Fat Duck in Bray, Berkshire, earlier this year.

Blumenthal was forced to close the three-Michelin star restaurant on 24 February after dozens of diners reported falling ill with diarrhoea, vomiting and headaches. Today the Health Protection Agency put the final total of people who fell ill at 529 – 15 per cent of diners over a six-week period.

Releasing a final report into its investigation, the HPA said that the illness had been caused by norovirus, a winter vomiting bug, most likely to have been passed on in shellfish from Essex infected with sewage from local people who had experienced an earlier outbreak. A large number of diners who fell ill had eaten the oysters and three other catering clusters of norovirus were traced back to the same source.

Blumenthal’s spokeswoman expressed relief that the illness had not been caused by food poisoning. However the HPA’s report made several criticisms of the Fat Duck, including its food handling and treatment of ill employees, who may have spread the virus by returning to work too soon.

Delayed for months by the outbreak of swine flu, the 47-page report clears up one of the great mysteries of British cooking: what stuffed the Fat Duck. On February 28 Blumenthal revealed he had shut the restaurant – voted the second best in the world this year – and called in environmental health officers. On March 12 the restaurant re-opened without (as The Independent revealed) shellfish on the menu. The closure cost the Channel 4 star an estimated £400,000.

“The organism responsible was norovirus which was probably introduced via shellfish,” the HPA said.

“Oysters were served raw; razor clams may not have been appropriately handled or cooked; tracing of shellfish to source showed evidence of contamination and there have been reports of illness in other establishments associated with oysters from the same source.”

It said the outbreak lasted from January 6 to February 22 because of ongoing transmission at the restaurant, which may have occurred through contamination of prepared food, by person-to-person spread between staff and diners or both. The HPA said: “Several weaknesses in procedures at the restaurant may have contributed to ongoing transmission including: delayed response to the incident; staff working when they should have been off sick and using the wrong environmental cleaning products.

“Delays in notification of illness may have affected the ability of the investigation to identify the exact reason for the norovirus contamination.”

A spokesperson for the Fat Duck said: “We are glad that the report has finally been published and draws a conclusion to the closure of the Fat Duck and more importantly that the norovirus has been identified as the cause and not due to any lapse in our strict food preparation processes.

“We were affected by this virus during a national outbreak of what is an extremely common and highly contagious virus. I would like to reassure our guests that they can continue to visit us with total confidence.”