The popular Mexican restaurant chain Wahaca has closed nine of its branches due to a suspected outbreak of norovirus in which hundreds of people have been affected.
The chain voluntarily closed stores after 205 staff and 160 members of the public fell ill last week, and initial tests suggested that norovirus was the cause.
Deborah Turbitt, London deputy director for health protection for Public Health England, 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.
The closed restaurants included sites at Canary Wharf, Oxford Circus, Soho, White City and the chain's flagship branch at Covent Garden.
Wahaca was opened by the British chef Thomasina Miers after she won TV's MasterChef in 2005.
Often referred to as the winter vomiting bug, norovirus causes severe diarrhoea and vomiting, although the illness is generally short lived and people tend to return to full health within a couple days.