Close to 300 of the sandwich and ready meal maker’s workers had tested positive for COVID-19.
Ireland-headquartered Greencore said in a statement: “As a result of the rising number of Covid-19 cases in the Northampton area, we took the decision to start proactively testing all of the colleagues at our Northampton site.
“We can confirm that a number of colleagues have tested positive for the virus and are now self-isolating.”
It said operations were continuing to run as normal.
Initially, 79 cases were detected, after Public Health Northamptonshire advised workers to get tested, before Greencore's subsequent private testing of 1,300 staff led to 299 positive results, with 400 pending.
Greencore said all of its sites have “wide-ranging social distancing measures, stringent hygiene procedures and regular temperature checking in place, and we are doing everything that we can to keep our people safe”.
Shares in the company fell 3% to 125.5p on Friday morning.
Analysts at broker Peel Hunt said: "We see a limited direct effect, as long as the plant is allowed to continue operations."