More than 300 people have fallen ill following a suspected outbreak of norovirus at the Wahaca restaurant chain, Public Health England (PHE) has said.
The Mexican restaurant chain had shut nine branches because of the infection, and four remain closed.
PHE has begun an inquiry after hundreds of staff and customers fell ill.
A spokeswoman for Wahaca apologised to those that had been unwell, saying it was an "unprecedented incident".
Wahaca was founded by 2005 MasterChef winner Thomasina Miers.
She said in 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."
Deborah Turbitt, London deputy director for health protection for PHE, said the source of the outbreak was yet to be determined.
She said: "We have so far been made aware of 205 staff and 160 members of the public reporting illness, however these are unconfirmed cases."
You might also like:
- White City
- Canary Wharf
- St Paul's
- Oxford Circus
- Covent Garden
Soho, Oxford Circus and Covent Garden remain closed. The Canary Wharf branch is shut but hopes to re-open on Thursday.
The restaurant chain voluntarily closed the affected restaurants and ensured staff members who had reported illness remained off site until their symptons had ceased for at least 48 hours, the statement said.
"We are incredibly sorry that people have been unwell," it read.
"In the nine years since we first opened Wahaca we have never had such an unprecedented incident, and we are doing everything we can to get to the bottom of how this may have happened."
"I was really cold and profusely sweating"
Simon Twinn ate at Wahaca in Soho, central London, on Saturday night with his girlfriend and some friends.
Speaking to BBC London Radio, he said: "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 after that.
"I was really cold and profusely sweating.
"We didn't think it was anything to do with the food until later that night my girlfriend started throwing up and getting stomach cramps.
"I contacted my friends and they were going through exactly the same thing."
"I started getting a really bad stomach ache"
Alex Gammage, 24, told BBC London she had eaten lunch with three friends at Wahaca at One New Change near St Paul's on 27 October.
"On Friday night I started getting a really bad stomach ache, like a knot in my stomach, and then about 30 minutes later I started to get sick and had diarrhoea. And that lasted the whole weekend.
"It was literally every 30 minutes. I couldn't even keep water down without being sick.
"It wasn't until Monday that I could eat anything and that was soup and its only been in the last day or so that I have started to get my appetite back."
She added that one of her friends had brought her baby along to the restaurant.
"The baby is only seven months old so if she had caught it, that would have been really scary."
Norovirus and its symptoms
- About 12 to 48 hours after becoming infected, the norovirus causes a sudden onset of nausea followed by projectile vomiting and diarrhoea
- Some people may have a fever, headaches and aching limbs
- Most people make a full recovery within one to two days, but the very old and very young risk becoming dehydrated which may require hospital treatment
- The virus is easily transmitted from one person to another by contact with an infected person or through contaminated food or drink, or touching contaminated surfaces or objects
- There is no treatment other than to let the illness run its course