A suspected outbreak of norovirus at Wahaca has caused more than 300 people to fall ill, according to Public Health England (PHE).
The Mexican eatery ‘voluntarily’ closed nine of its branches because of the infection, and four remain shut.
Chichester’s Wahaca branch has been unaffected by the outbreak.
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’.
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.”
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.”
The affected restaurants are: