The number of diners said to have fallen ill after eating at celebrity chef Heston Blumenthal's Fat Duck has risen to 400.
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.
"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."
A series of rigorous health tests are being carried out but these have so far failed to identify the cause of the scare.
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.
Tests are being carried out on all of Blumenthal's employees and samples were also being taken from diners who reportedly fell ill after a meal.
A risk assessment of all food storage, preparation and cooking processes was also taking place and experts were examining the possible contamination of food by germs - either bacterial or viral.
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