"We took our own precautionary measures – voluntarily closing affected restaurants, carrying out anti-viral deep cleaning at all of our restaurants, whether affected or not, and ensuring that any staff member who had reported illness remained off site until their symptoms had ceased for at least 48 hours, "they added.
Five restaurants have since reopened.
U.K. watchdog Public Health England told The Guardian newspaper that it had launched an investigation after 205 employees and 160 members of the public fell ill.
"Currently the source of the outbreak is not confirmed and this remains under investigation, " Deborah Turbitt, London deputy director for health protection for PHE, told The Guardian.
Norovirus causes diarrhoea and vomiting, the UK's National Health Service website says, adding that symptoms appear one to two days after patients become infected and typically last for up to two or three days.
Public Health England was not immediately available for comment when contacted by CNBC.
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