The celebrity chef is now unlikely to reopen the restaurant this week as further tests are being carried out.
Blumenthal, chef-proprietor of the iconic restaurant in Bray, Berkshire, closed the Fat Duck last Tuesday (24 February) after up to 40 diners were afflicted with a mystery illness resulting in vomiting and diarrhoea.
However, the Health Protection Agency (HPA) said cases reported to the restaurant had risen since media coverage of the outbreak.
Environmental health inspectors were called in to scour the kitchens at the Fat Duck for signs of malpractice last Wednesday but test results came back clear, with Blumenthal insisting that "absolutely no food poisoning organisms have been found".
Dr Graham Bickler, regional director at the HPA, said the outbreak was "very complex".
"We are working closely with the restaurant and with colleagues in the royal borough's environmental health team to explain what happened and to ensure that the risks of it happening again are reduced as much as possible," he said.
Scientists are now conducting a "risk assessment of all food storage, preparation and cooking processes" at the Fat Duck and are examining food samples and specimens from diners and all members of staff.
Laboratory tests will also be carried out into possible contamination of food by bacterial or viral germs, which might have occurred at any time from "before supply to the restaurant to reaching the plate".
Blumenthal said he was "upset" and "frustrated" by the health scare.
He has been forced to cancel more than 500 reservations at the Fat Duck, resulting in a major financial blow to the restaurant, estimated at £100,000 a week.
By Kerstin Kühn
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