Almost 200 staff at an IT logistics company have tested positive for Covid-19, forcing the firm to temporarily close its site to contain the outbreak.
A total of 190 workers out of 795 at Ingram Micro Services in Norwich have had coronavirus in the past 28 days, Norfolk County Council said.
Its site at an industrial estate will close for 10 days from Thursday.
Ingram Micro Services said the welfare of its team was its primary concern and it would offer support as needed.
Since Monday, staff working at Vulcan Road North have been offered testing by the county council's public health team.
Workers who have not been tested in the past 90 days should arrange a test, whether or not they have had symptoms, Public Health England said.
'Break the chain'
Dr Louise Smith, Norfolk's director of public health, said she welcomed the firm's "swift and decisive action".
"It's vital that we act swiftly to break the chain of transmission and prevent any further spread among this workforce or more widely into the community," she added.
"All those who have already tested positive are now isolating and all the rest of the staff will now also be asked to isolate for 10 days."
Kevin Coleman of Ingram Micro Services said it had "invested heavily" in preventative measures at its Norwich site and changes to its working practices had been in place for many months.
Staff are being offered support by the city council and the county council's community resilience team, including with access to food, medication and emergency financial support.