A sandwich-making factory where hundreds of workers tested positive for Covid-19 has been temporarily closed.
Workers at M&S supplier Greencore in Northampton, where almost 300 people were found to have the virus last week, will go into self-isolation.
It comes as Northampton has been added to a government watchlist as an "Area of Intervention".
Director of Public Health Northamptonshire, Lucy Wightman, said: "The factory will close voluntarily."
She added that "most employees and their direct households will be required to isolate at home for two weeks".
The factory will now undergo a full deep clean.
Northampton has seen a marked increase in cases, with its incidence rate hitting 125.1 per 100,000 people during the week ending 13 August.
Testing data and analysis from the Joint Biosecurity Centre shows the spike is "almost solely down" to the outbreak at Moulton Park-based Greencore, which employs 2,100 people.
Mrs Wightman said Northampton has been "experiencing a high number of cases over the last four weeks", adding the current situation is "fragile".
What is an Area of Intervention?
Public Health England publishes a weekly watchlist of local authorities as part of its surveillance report of coronavirus infections.
It has three categories for local councils: areas of concern, areas for enhanced support and areas of intervention.
An area of intervention - the highest level - is an area where there is a "divergence" between the local measures in place to control the spread of coronavirus and national restrictions across England.
The BBC has seen a message sent to staff at Greencore that read: "We know this will come as concerning news for all our colleagues.
"We will therefore send further updates to you on the action you need to take shortly. Please stay safe."
A spokesman for Greencore said the decision to close from the end of Friday followed talks with the Department of Health and Social Care and Public Health England.
"This decision will allow all remaining colleagues at the site to self-isolate as a precautionary measure, and has been taken as part of the region's ongoing efforts to reduce the spread of the virus and keep our colleagues safe," he said.
He added "a proportion of production from the Northampton facility" has been moved to other Greencore sites.
At a press conference on Friday afternoon, Northampton Borough Council's leader Jonathan Nunn said local authorities had already had "significant support from the government", including from military planners.
Mrs Wightman said a second round of testing at the factory, where 287 workers tested positive, had not been completed.
Asked why the factory was not closed when the outbreak was first uncovered, she said: "All evidence is that the setting is not the risk factor. The Secretary of State has now decided to take that step."
She added Greencore was working to produce figures on many people would be affected, with family members and those also living with workers in houses of multiple occupation also having to self-isolate.
Mr Nunn said there was a "moveable feast" of measures still available and that the aim was still to avoid a local lockdown.
Traders in Northampton town centre questioned why it took a week from the Greencore positive tests being revealed before the factory was closed.
Wes Souter, who owns Steffans Jewellers, said he feared "we are going to be locked down" and said it was a "horrendous situation".
Owner of the Market Square based Cafe Track, Tom Cliffe, added: "I don't want to be scaremongering, but it is a clear message that we are one step from lockdown."
A government spokesman said Health Secretary Matt Hancock would be introducing regulations to ensure the period of self-isolation was legally enforced.
He said: "Anyone who leaves isolation prior to the two-week period ending without reasonable excuse will be subject to fines."