Hundreds of people have fallen ill after a suspected norovirus outbreak across a chain of Mexican-style restaurants.
More than 300 staff and customers may have been affected, Public Health England (PHE) said.
Wahaca, founded in 2005 by MasterChef winner Thomasina Miers and businessman Mark Selby, was forced to close nine restaurants across the UK after the reported outbreak on Wednesday.
Norovirus, also known as the "winter vomiting bug", causes diarrhoea and vomiting but is not life-threatening. It is spread by infected particles of faeces or vomit entering your system, and is usually transferred in hospitals or unclean public spaces.
The Canary Wharf, Covent Garden, Soho, and Westfield branches of the chain all remained shut on Thursday, but a spokesperson for the group said the Canary Wharf site was expected to reopen later today.
Deborah Turbitt, London deputy director for health protection for PHE, said they were unsure where the outbreak had originated. She confirmed to BuzzFeed News they had been "made aware of 205 staff and 160 members of the public reporting illness, however these are unconfirmed cases".
"We are working closely with environmental health officers and the restaurant chain to investigate," Tubitt said.
The cases appeared to have spread across the UK.
Elliot Grieve, 30, said he and his partner had been "very, very, sick" after visiting the Edinburgh Wahaca branch on Saturday.
"I'm off work still – they don't want norovirus spreading around the office, obviously," he told BuzzFeed News. Grieve, who lives in Edinburgh, said the experience had been "horrible".
Another person affected was Jo, the partner of Edinburgh resident Ryan Gilbert. "She and her two friends went to the new Wahaca in Edinburgh for lunch on Saturday," he told BuzzFeed News. "By Sunday night all three of them had fallen ill and were vomiting. Knocked them all out for around 48 hours."
Gilbert, 37, who lives in Edinburgh, said Wahaca had been in touch and were "looking at the issue with a view to some compensation".
Other customers have not been so fortunate. Emily Farrow, a doctor based in Stevenage, said she and a group of friends had visited the Brighton venue last month.
Farrow, 24, said she and her boyfriend contracted severe symptoms after eating at the branch on 3 October. "We spent the next 18 hours both suffering with profuse vomiting and diarrhoea and unable to eat or work the next day," she told BuzzFeed News.
Despite contacting the chain and listing her symptoms, she found staff "reluctant to take any responsibility". "I was really shocked," she said.
When contacted by BuzzFeed News, a spokesperson for the company declined to comment on individual cases.
But Wahaca founders Mark Selby and Thomasina Miers said in a statement: "We assessed each case and when it became clear they were not isolated incidents, we got in touch with relevant officials at Public Health England and Environmental Health Officers.
"In tandem with that, we took our own precautionary measures – voluntarily closing affected restaurants, carrying out anti-viral deep cleaning at all of our restaurants, whether affected or not, and ensuring that any staff member who had reported illness remained off site until their symptoms had ceased for at least 48 hours."