The “culinary alchemist” has temporarily shut the restaurant as a precaution after 40 customers came down with a stomach bug.
The Health Protection Agency (HPA) said the number of cases reported, going back to late January, has risen since media coverage of the outbreak.
The HPA’s Thames Valley unit is working with the Royal Borough of Windsor and Maidenhead Environmental Health Department to investigate the outbreak of diarrhoea and vomiting.
Dr Graham Bickler, regional director of the HPA south east region, said: “This is a very complex outbreak.”
Blumenthal had hoped for the all-clear from environmental health officers on Wednesday but the inconclusive investigation meant he was unable to reopen the Michelin-starred restaurant in Bray, Berkshire.
The Fat Duck remained closed yesterday.
Tests were being carried out on all of Blumenthal’s employees and samples were also being taken from diners who reportedly fell ill after a meal.
Blumenthal, who opened The Fat Duck in 1995, found fame with his innovative cooking style — he researches the molecular compounds of dishes to achieve a greater understanding of taste.
His scientific approach to flavours meant The Fat Duck was voted “the best place to eat on earth” by Restaurant magazine in 2005. It is currently second only to Spain’s El Bulli.