Supermarket depot workers and suppliers will be exempt from self-isolation rules under a plan to avoid food supply shortages caused by the UK's Covid-19 "pingdemic".
Workers in 2,000 warehouses and supermarket distribution centres will be exempt from quarantine rules if they are a close contact of a positive coronavirus case.
Those workers, regardless of vaccination status, will be asked to take daily coronavirus tests at work instead of self-isolating.
A separate scheme allows a limited number of vaccinated essential workers in other industries to take daily Covid-19 tests to keep attending work if they have received an alert.
This scheme covers sectors including transport, emergency services, border control, energy, digital infrastructure, essential chemicals, nuclear power, waste, the water industry, essential defence outputs and local government.
Some supermarkets are facing shortages of products, mainly those in demand during hot weather, and some petrol stations have had to close after workers were told to isolate because of contact with an infected person.
With cases rising to nearly 50,000 a day in the UK, hundreds of thousands of people have been advised, or "pinged", by the National Health Service's contact-tracing app to go into isolation for 10 days.
A record 618,903 people were told to isolate for ten days by app last week, a rise of 16 per cent on the previous week.
Environment Secretary George Eustice said the exemption will begin for 15 "priority testing sites" on Friday, before it is expanded to "several hundred" supermarket depots and food manufacturers over the coming weeks.
He urged shoppers not to panic buy as this would lead to further shortages.
"We will never take risks to our food supply," he told Sky News.
"The last 18 months have demonstrated that we have a highly resilient food supply chain. There are sufficient food supplies in the system and people can and should shop as normal."
While supermarket workers will receive a blanket exemption, employees in several other key sectors need to apply to the government to continue attending work.
Mike Clancy, general-secretary of the Prospect trade union representing sectors including science and engineering, said businesses were weighing up whether to participate in the scheme given that self-isolation rules are due to be relaxed on August 16.
He said the scheme was complicated and excluded thousands of workers in support roles.
"Just listing named individuals, which seems to be at the core of this proposal, won’t work," he told BBC's Radio 4 Today programme.
"It hasn't been thought through and there's been a vacuum of leadership here."
Mr Eustice said the government would not relax self-isolation rules for all fully vaccinated workers before August 16 because the government needed to slow down the "velocity in which this infection is spreading".
"We have to keep a very close eye on hospitalisations," he said.
"We need to make sure we don’t have this growing too quickly and that’s the aim of this."
Meanwhile, the founder of one of the country’s largest food producers said the industry was at “crisis point”.
Ranjit Singh Boparan, who is known as the "Chicken King" and owner of the 2 Sisters Food Group, said the pingdemic had come on top of Brexit-related problems.
"No one could possibly have predicted that this toxic cocktail would come together at this time," he told MPs.
“It started with the pandemic – and in the last week or so with pingdemic, but since May this year the operating environment has deteriorated so profoundly I can see no other outcome than major food shortages in the UK.
"Supply of chicken and turkey is under threat. Our retail partners and the wider supply chain have worked together closer than ever before to ensure we retain food supply and this is of huge credit to everyone. But we are at crisis point.”
Health Secretary Sajid Javid said the government was trying to curb the spread of the virus.
"As we manage this virus and do everything we can to break chains of transmission, daily contact testing of workers in this vital sector will help to minimise the disruption caused by rising cases in the coming weeks, while ensuring workers are not put at risk," he said.
He played played down fears of shortages, saying problems were not widespread.
Sainsbury's, Britain's second largest grocer, said customers might not be able to find the exact product they want.
"Large quantities of products are being delivered to stores daily and our colleagues are focused on getting them on to the shelves as quickly as they can," a representative said.
Asda, the industry's third largest player, said although more staff were being forced to isolate, the level of absences was well below that at the height of the pandemic last year.
"We’re also not in a position where we would have to close any stores," a representative said.
Product availability was "largely OK across the board", although some stores were short of warm weather lines: beer and soft drinks, bottled water, ice cream and salad leaves.
But smaller rival Iceland said it had closed some stores because of staff shortages.
BP said it had to temporarily close some petrol stations because of a lack of fuel, with a shortage of heavy goods lorry drivers exacerbated by Covid-19 isolations.
Official data showed the app had told nearly 620,000 people to isolate in England and Wales in the week to July 14.