Wahaca has closed nine restaurants after more than 200 staff and some 160 diners fell ill.
The Mexican restaurant group voluntarily closed the London sites after a suspected norovirus outbreak.
According to its website the closed restaurants include Canary Wharf, Covent Garden, Oxford Circus, Soho and White City, all in London. Five sites have since reopened after specialists cleaning.
A spokesperson told the Daily Mirror: "We can confirm a number of our employees and customers have reported becoming ill over the past week.
"Initial tests suggest that the cause of this illness was norovirus. Following these reports, Wahaca voluntarily closed nine sites."
"After specialist deep cleaning of all sites, whether affected or not, and with the co-operation of relevant authorities we have reopened five of those sites and continue to monitor the issue closely in order to ensure it is contained and resolved.
"As ever our main priority is to ensure that all of our customers and our valued team members are safe and healthy."
Public Health England said that the source was yet to be determined, but that it was suspected to be norovirus.
London deputy director Deborah Turbitt 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."
Norovirus causes diarrhoea and vomiting but those affected are typically over it in 48 hours.
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