A factory must partially close after 75 poultry processing workers test positive for Covid-19.
Tests were carried out on 347 employees at Banham Poultry in Attleborough, Norfolk, after seven of them caught the virus.
County public health director Dr Louise Smith said it “was now considered a significant national epidemic”.
Staff in the cutting area of the plant were asked to isolate themselves with their families.
Dr Smith said: “The measures were taken because we believe it is the best way to protect the general public from new infections. ”
The source of the outbreak is not yet known, but a geographic or urban lockdown was not expected at this point, Dr Smith said.
Other areas of the processing plant are said to remain open for the time being, although the outbreak is under review by the central government on Thursday.
At least one factory worker has been taken to hospital but his condition is not known.
Banham Poultry worked with the council’s public health department and had “voluntarily agreed” to shut down part of the plant.
A thorough cleaning of the cutting room, where the poultry is cut and packaged, will now take place.
Only personnel working in one area of the plant were tested while the larger plant, which employs around 1,100 people, was opened for production.
Dr Smith said his department has sought expert advice from Defra and the Food Standards Agency, as well as biosafety departments.
She said they were considering talking to other poultry factories in the area.
The east of England processes 7% of the country’s poultry.
Factories across the UK are working to minimize the risk from the coronavirus, but despite this, hundreds of workers have been infected.
Northampton’s Greencore sandwich shop, where nearly 300 workers tested positive last week, has now reopened as a phased start-up.
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