About 400 customers at the restaurant have reported feeling unwell
The number of people who have reported feeling ill after eating at Berkshire's Michelin-starred restaurant the Fat Duck has risen to 400.
Celebrity chef Heston Blumenthal closed his restaurant in Bray more than a week ago after 40 customers fell ill.
Mr Blumenthal said tests had ruled out food poisoning.
In a statement the Health Protection Agency (HPA) said cases reported to the restaurant had risen since media coverage of the outbreak.
Boxing promoter Frank Warren said he fell ill after eating at the restaurant with friends and would welcome an apology.
"I really [did] enjoy the experience of it all," he said.
"Heston's obviously a very talented guy, I watch his programmes and I hope he gets it sorted out.
"The people I was with, all of them are still suffering."
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.
The restaurant remained shut on Friday.
Dr Graham Bickler, regional director of the HPA south east region, said: "This is a very complex outbreak.
The tasting menu at the Fat Duck costs £130
"We are working closely... to explain what happened and to ensure that the risks of it happening again are reduced as much as possible."
Samples have been taken from foodstuffs, people who have reported feeling ill, and staff and work surfaces in the search for the cause of the illness.
Mr Blumenthal, 42, has spoken of how the outbreak has been an awful setback - the chef has cancelled more than 500 bookings.
He said the entire menu had been tested and all results had proved negative. Final tests are being carried out to see if an airborne virus caused the sickness.
Charlie Bedford-Forde went to the Fat Duck to celebrate his birthday in January.
His brother and mother became ill and believes the cause could be a winter vomiting bug.
"While we all had a fantastic time, my brother and mother fell ill with diarrhoea and vomiting, for five days after," Mr Bedford-Forde added.
"My brother is a chef in a Michelin-starred restaurant in Kent and therefore provided samples to the doctor which were tested and came back totally negative."
Councillor Phillip Bicknell, the council's lead member for public protection, said: "We are committed to the welfare of our residents and visitors to the restaurant and need to ensure the investigation is thorough."
Diners at the Fat Duck - one of only three restaurants in the UK with three Michelin stars - can experience dishes such as snail porridge or scrambled egg and bacon ice cream.
Opened in 1995, The Fat Duck was voted "the best place to eat on Earth" by Restaurant magazine in 2005.