TV chef Heston Blumenthal said he was baffled by what may have caused a health scare that forced him to close his award-winning restaurant.
The "culinary alchemist" temporarily shut The Fat Duck as a precaution while tests are carried out after around 40 customers were taken ill over the last couple of weeks.
Blumenthal said that food poisoning had been ruled out but he took the decision to shut the Michelin-starred restaurant in Bray, Berkshire, while an investigation is carried out.
Tests by the celebrity chef's own food safety consultants and environmental health officers from Windsor and Maidenhead Borough Council have so far failed to solve the mystery.
He said: "The whole thing is really upsetting and perplexing. We have done so many tests but nothing has shown up. We called in the environmental health and I took the decision to close the restaurant. All of our checks were clear but if there is a problem then the restaurant has a duty to try and find out what it is."
The chef said that experts believe it could be a non-specific virus that is going around.
He added that some staff who were on a recent holiday to Venezuela were tested in case they had picked up any tropical virus but results came back negative.
He said: "All of our systems, processes, supply chain and general hygiene have all been cleared. We started with the obvious stuff and then moved to the less obvious stuff. But it is still a mystery. I don't want anyone leaving my restaurant with such as a headache so this is really upsetting."
Blumenthal said that he has had to cancel 450 bookings but that customers had been really understanding.
He said the results of all tests should have been completed by Monday and he hopes to reopen the restaurant on Wednesday.
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