The outbreak at the facility in Great Witchingham, Norfolk follows 72 positive tests at another of the company's plants in Suffolk.
Workers on the late shift at the factory were handed letters on Monday telling them to go home and self-isolate for two weeks after 600 staff were tested.
Staff have now been ordered to stay at home for 14 days if they have not tested positive, or if they are yet to be tested.
Anyone testing positive needs to isolate for 10 days.
The mass testing is aimed at identifying workers who are infected, but asymptomatic.
The outbreak in Suffolk was first identified in September and blamed in part on workers sharing car rides to the factory from their homes.
As of October 15 there had been 72 positive cases at Bernard Matthews' food processing facility in Holton near Halesworth, Suffolk County Council said.
The authority said in a statement: "We can confirm that after precautionary testing began at Bernard Matthews' site in Great Witchingham on October 15, results received so far have shown a total of 75 Covid-19 positive cases with over 600 staff tested at the site, and the results of the final tests still being processed.
"Results showed that the majority of positive cases so far worked on the afternoon shift at the site, leading Public Health to advise Bernard Matthews that the entire shift be instructed to self-isolate.
"It is advised that staff must self-isolate for 14 days, if they have not tested positive or not been tested.
"Staff must isolate for 10 days if they have had a positive test result, and the households of positive cases, including children, need to isolate for 14 days."
In Suffolk, Bernard Matthews brought in Covid-19 bus marshals on its free staff transport service, created a one-way system and staggered break times as part of its response.