About 400 customers at the restaurant reported feeling unwell
An investigation into what caused 400 people to feel ill after eating at Berkshire's Michelin-starred restaurant the Fat Duck could last several weeks.
Celebrity chef Heston Blumenthal closed his Bray restaurant at the end of last month after 40 customers fell ill. He said tests ruled out food poisoning.
The Health Protection Agency (HPA) said it "needed to do a thorough food diary with each individual".
But it added the restaurant could reopen while the probe continued.
The HPA's Thames Valley Health Protection Unit is working with the Royal Borough of Windsor and Maidenhead's environmental health department to investigate the outbreak of vomiting and diarrhoea.
Samples have been taken in the search for the cause of the illness.
A spokeswoman for the HPA South East Region said: "This is a complex investigation - we are working with 400 possible cases."