Mexican high-street food chain Wahaca closed nine of its restaurants after a suspected outbreak of norovirus left more than 300 people ill.
Four of the nine restaurants remain shut because of the infection, reported the BBC.
Public Health England (PHE) has launched an inquiry into the suspected outbreak, which has affected 205 staff and 160 members of the public.
Its restaurants in Soho, Oxford Circus and Covent Garden remain closed, and the Canary Wharf branch was also shut but re-opens today.
Other branches that have since re-opened are: Manchester, Brighton, Cardiff, White City and St Paul's, the BBC reported.
A Wahaca spokesperson apologised to those who had been taken ill and said the incident was "unprecedented".
Thomasina Miers, the former MasterChef winner who founded Wahaca in 2005, said in a statement with co-founder Mark Selby: "Last week a number of our staff and customers were struck down by what is suspected to be the winter vomiting bug, norovirus.
"We assessed each case and when it became clear they were not isolated incidents, we got in touch with relevant officials at PHE and Environmental Health Officers."
The source of the outbreak hasn't yet been determined, according to PHE, and a Wahaca spokesperson said "we are doing everything we can to get to the bottom of how this may have happened".
A man who ate at Wahaca in Soho on Saturday night with his girlfriend and some friends shared details of their unpleasant symptoms with BBC London Radio.
"We were fine on Sunday. On Monday I started needing the toilet quite badly to say the least and then vomiting started a few hours afters that," said Simon Twinn. "I was really cold and profusely sweating."
Twinn didn't think it was anything to do with the food until later that night when his girlfriend also started throwing up and getting stomach cramps.
"I contacted my friends and they were going through exactly the same thing," he added.
Along with nausea, vomiting and diarrhoea, other symptoms of norovirus include a fever, headaches and aching limbs.
It's easily transmitted from person to person through contact with someone who is infected, by consuming contaminated food or drink, or touching contaminated objects.
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