Looking back, diners really would have been better if they had stuck with that Heston Blumenthal classic, snail porridge, or for that matter, gone à la carte. Those, however, who opted for the £130 "tasting menu" were the unfortunate ones who fell victim to an outbreak of food poisoning that embarrassed Blumenthal's world-famous restaurant. The shellfish were to blame, it seems.

In detail that would be excruciating to the lowliest of cooks, the report from the Health Protection Agency flagged up the dishes "oyster, passion fruit jelly, lavender" and "sound of the sea" (which was also flavoured with oysters) as the culprits that probably caused more than 500 fans of fine dining, including stars of the sporting and entertainment world, to fall ill. For the record, the "jelly of quail, langoustine cream, parfait of foie gras" was also in the frame – though the evidence against it is less strong.

The basics of the HPA report are bad enough for a chef such as Blumenthal, who is not only renowned in the business for his innovative methods – he follows the principles of "molecular gastronomy" – but well known for his fastidiousness in his kitchens.

It describes how 15% of customers to the restaurant in January and February this year reported illness. The restaurant in Bray, Berkshire, closed and the food inspectors moved in.

The organism responsible for this outbreak of vomiting, diarrhoea and stomach aches among the rich and famous was the norovirus, which, according to the report, was "probably introduced by shellfish".

That didn't take any great detective work – more people who were ill reported that they had eaten the oysters, which can be contaminated with human sewage, especially in the winter months.

HPA officials traced the oysters back to their source in Essex. It found "evidence of contamination" and reports of illness by diners who had visited other establishments that also used the shellfish.

The report goes on to claim that the outbreak continued for at least six weeks because of "ongoing transmission at the restaurant". This may have occurred through "continuous contamination of foods prepared in the restaurant, or by person-to-person spread between staff and diners, or a mixture of both".

It also reports "several weaknesses in procedures at the restaurant" including a delayed response to the outbreak and the problem of staff who had been affected by the illness continuing to work.

But the bare bones of what they call one of the largest outbreaks of norovirus reported in medical literature do not tell the half of it.

The report says the HPA was informed of the problem in late February, though it claims the restaurant had been aware of it since early January.

Once they knew about it, health officials began contacting people who had dined at the Fat Duck recently, and all 50-odd members of staff. In all, 529 reported illnesses. Typically they became ill in 24 to 48 hours and stayed sick for three days. They reported diarrhoea, vomiting and nausea. Several suffered the indignity of providing stool samples for the investigation. Seventeen members of staff reported symptoms of gastrointestinal infection, and six said they worked while unwell. One even vomited in the restaurant toilet, though happily it was closed at the time. Nine returned to work before being clear of symptoms for 48 hours – against guidelines, according to the report.

Two samples of food retrieved from the restaurant had, the report said, "indicators of poor hygiene" – cooked razor clams and langoustine cream, in which E coli, an indicator organism for faecal contamination, was found.

But attention focused on the favourite dish from the tasting menu – "oyster, passion fruit jelly, lavender" and the slightly less popular "sound of the sea".

Many of those who eat at the Fat Duck choose the tasting menu because it gives them the chance to sample a selection of Blumenthal's strange creations.

The inspectors began to look closely at the oysters the Fat Duck used. According to the report, they come from a supply in Essex. Inspectors identified three other outbreaks of norovirus "potentially linked to this same supplier". Sampling of oysters harvested from the implicated site in the river Colne in March tested positive for norovirus (genogroups I and II).

The HPA's south-east regional director, Graham Bickler, said the "complex" inquiry confirmed the "well-known risks that raw shellfish pose".

He said: "Oysters and other shellfish can become contaminated with norovirus originating from human sewage, especially during winter months.

"Individuals infected with norovirus can readily transfer the virus on to foods they prepare. The virus will remain viable and capable of causing illness in those foods that are not subsequently cooked. The more intensively that food is handled the more likely it is to become contaminated by infected food handlers."

Blumenthal, who friends say is "bruised" by the experience, was not commenting today, but a spokeswoman said the report showed the restaurant was affected by an "epidemic not isolated to the Fat Duck".

She insisted that the outbreak was caused by shellfish contaminated at source rather than by hygiene standards or food preparation techniques at the restaurant, and Blumenthal has called in Hugh Pennington, emeritus professor of bacteriology at the University of Aberdeen, for advice.

The restaurant reopened in March and, as ever, is booked out for the next two months. Oysters, for the foreseeable future, remain off the menu.

Verdict

• There was a large outbreak of food poisoning among diners at the Fat Duck in January and February 2009, with more than 500 reporting illness – more than 15% of those dining there during this period.

• The organism responsible was norovirus which was probably introduced via shellfish.

• 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.

• The outbreak continued for at least six weeks because of ongoing transmission at the restaurant.

• Several weaknesses in procedures at the restaurant may have contributed to transmission, including: delayed response to the incident; staff working when they should have been off sick; and using the wrong cleaning products.

• Delays in the notification of illness may have affected the ability of the investigation to identify the reason for the norovirus contamination

Source: The Health Protection Agency