All late-shift employees at a turkey processing factory have been ordered to self-isolate after 75 workers tested positive for Covid-19.
The decision by Norfolk Public Health follows precautionary testing of staff at Bernard Matthews in Great Witchingham, Norfolk.
The 75 positive cases were among about 600 results returned so far.
A cluster of unrelated cases was found earlier this month among staff working at the factory.
"Results showed that the majority of [people with] 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," said a spokesman.
The company operates two shifts: an early which runs from 6am to 2pm and a late from 2pm until 10pm.
Norfolk County Council said employees did not switch between shifts.
Staff have been told to self-isolate for 14 days, even if they have not tested positive or not been tested at all.
Bernard Matthews is part of the Two Sisters food group, which has been asked for comment.
Since 15 October, precautionary tests have also been carried out on 1,000 staff employed at the firm's Suffolk plant, following an outbreak there in September.
The factory introduced a range of measures, including new entrances to aid one-way movement of staff, breaks being staggered and the introduction of bus marshals on the firm's free staff transport.