All late-shift employees at the Bernard Matthews headquarters in Norfolk have been told to self-isolate by health bosses in the county after 75 workers tested positive for coronavirus.
The decision was made by Norfolk Public Health after all staff at the factory in Great Witchingham near Norwich were tested as precaution.
Out of the 600 test results returned so far, 75 came back as positive - with the vast majority of those coming from people who worked on the evening shift.
"We can confirm that after precautionary testing began at Bernard Matthews’ site in Great Witchingham on Thursday (15th October), results received so far have shown a total of 75 Covid positive cases with over 600 staff tested at the site, and the results of the final tests still being processed," a Norfolk County Council spokeperson said.
"Results showed that of the majority of positive cases so far worked on the afternoon shift at the site, leading to Public Health to advise Bernard Matthews that the entire shift be instructed to self-isolate."
Late-shift staff have now been ordered to stay at home for 14 days if they haven't tested positive or haven't yet been tested.
Those who have tested positive have been told to isolate for 10 days, and their households must isolate for 14 days.
A small outbreak of unrelated cases was previously identified at the factory earlier this month, while testing has also been carried out on 1,000 staff at the firm's Suffolk site in Holton after 72 members of staff tested positive there.