The Mexican restaurant chain Wahaca shut nine of its restaurants after a suspected outbreak of the norovirus infection.
More than 300 people have fallen ill following the suspected outbreak of the illness, Public Health England (PHE) has said.
A spokeswoman told the Mirror: "Initial tests suggest the cause of this illness was norovirus.
"Following these reports, Wahaca voluntarily closed nine sites."
Five of the closed restaurants are in London.
Deborah Turbitt, London deputy director for health protection for PHE, said the source of the outbreak was yet to be determined.
She said: "Public Health England has been notified of a suspected norovirus outbreak at a restaurant chain.
"We are working closely with environmental health officers and the restaurant chain to investigate.
"We have so far been made aware of 205 staff and 160 members of the public reporting illness, however these are unconfirmed cases.
"Currently the source of the outbreak is not confirmed and this remains under investigation."
The restaurant chain was founded by MasterChef winner Thomasina Miers in 2005.
A spokeswoman for the restaurants said that specialists had cleaned all Wahaca restaurants and four affected sites were subsequently reopened, with a London restaurant expected to re-open for lunch service today.
According to its website, its restaurants in Oxford Circus, Canary Wharf, Covent Garden, Soho and White City remain closed.
The virus causes diarrhoea and vomiting and those affected generally return to full health after a few days.
The NHS advises calling a GP or the 111 helpline if symptoms persist.