At least 350 people and their households will have to self-isolate after 75 staff tested positive for coronavirus at a poultry plant in Norfolk, as Boris Johnson was urged to do more to get a grip on factory outbreaks.

The Banham Poultry site, about 15 miles south-west of Norwich, had been partially closed and all staff who worked on its cutting floor had been sent home, local health officials said.

The prime minister assured MPs in June that the prevalence of outbreaks in meat processing plants was being taken seriously by the government. However, when asked last week, Whitehall officials were not able to produce any data that even measured the scale of the problem.

On Wednesday, the chief operating officer of the Food Standards Agency reportedly said it was looking at about 40 outbreaks in factories in England. According to the industry magazine Food Manufacture, Dr Colin Sullivan acknowledged there was evidence of increased prevalence but he admitted he did not have reliable data to hand.

Responding to the announcement about the partial closure of the Banham site on Thursday, the shadow health secretary, Jonathan Ashworth, said: “This is more evidence that Boris Johnson’s test-and-tracing system simply isn’t ‘world beating’, as promised.

“It is vital detailed data is collected when testing and tracing, including place of work, so hotspots can be quickly identified and decisive action taken – including temporarily closing factories where necessary – by local public health officials.”

Norfolk officials declared the Banham outbreak on Tuesday, saying at that point that seven positive tests had been returned. That was increased to 46 the next day and 75 on Thursday as more test results were returned.

Norfolk county council said: “As a result of this infection rate, which is at about 22%, Banham Poultry has voluntarily agreed to close part of their site following advice from Public Health England and Norfolk Public Health.

“Those who have tested positive will be asked to isolate for 10 days. Those who have tested negative will be asked to isolate for 14 days.”

Parts of the factory remained open, with staff in those areas told to continue working, but their numbers are not known.

Norfolk’s director of public health, Dr Louise Smith, said: “This action has been taken because we believe it is the most effective way to protect the wider public from further transmission … There is no evidence of increased risk to the general public. The risk of infection from food products is very low.”

The Banham case is the latest in a string of outbreaks in food processing plants around the country. Last week, a Guardian analysis found at least 1,454 confirmed infections linked to such sites across England, Scotland and Wales, including several in recent weeks. More were expected to have occurred in Northern Ireland, but authorities there will not publish specific statistics.

Johnson told the Commons about two months ago: “We will certainly look into what is happening to meat processing … We need to get to the bottom of what is happening.”

He added: “Currently, two theories have been advanced to me: one is about the cold environment in the plants, which may be propitious to the virus, and the other is the possibility that staff are congregating in such a way as to spread the virus. We do not know what it is, but we are investigating.”

Banham Poultry has been approached for comment.