Emergency measures to protect food supplies will see thousands of workers at up to 500 critical sites potentially avoid the need to self-isolate if identified as a contact of a coronavirus case.

The move – along with a limited relaxation of self-isolation rules in other key sectors of the economy and vital public services – came as Boris Johnson faced mounting warnings about the impact of the “pingdemic”.

Under the plan to keep supermarket shelves stocked, daily testing will be offered as an alternative to self-isolation in important links in the food supply chain.

Health Secretary Sajid Javid said: “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.”

Retail bosses urged shoppers not to stockpile and said there is plenty of food, but businesses are being hit as staff are “pinged” by the app or contacted by NHS Test and Trace.

Supermarkets such as the Co-op said they are seeing availability issues with some products, but stressed that shortages are “patchy” across stores.

Priority testing sites – including the largest supermarket distribution centres – have already been identified for urgent implementation this week, with hundreds more planned next week.

It is absolutely vital that Government makes up for lost time and rolls out this new scheme as fast as possible

The move means workers who have received an NHS Covid-19 app alert to isolate or have been called by Test and Trace will be able to continue working if they test negative.

Helen Dickinson, chief executive of the British Retail Consortium, welcomed the new measure but said ministers must be prepared to take further action if necessary.

“It is absolutely vital that Government makes up for lost time and rolls out this new scheme as fast as possible,” she said

“Disruption is limited at the moment, and retailers are monitoring the situation closely.”

Environment Secretary George Eustice said: “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.”

The need for urgent action was underlined as the latest figures showed a record number of people in England and Wales were “pinged” as contacts by the app and told to self-isolate for up to 10 days.

NHS figures showed 618,903 alerts were sent to users of the coronavirus app in the week to July 14, a period before England’s restrictions were lifted and more social contact was allowed.

Former health secretary Jeremy Hunt said the Government risks “losing social consent” for isolation if it does not immediately bring forward the relaxation of quarantine rules for the fully vaccinated.

People in England who are fully vaccinated will not have to self-isolate if identified as a contact from August 16, nearly a month after restrictions on social mixing were lifted and at a time when cases have soared.

Alongside the measures to protect food supplies, the Government published guidance on Thursday night setting out limited exemptions for other critical workers.

Employees providing critical services would only be able to keep working and avoid self-isolation after being identified as a contact if they were named on a list kept updated by officials.

The exemptions – mainly in 16 sectors including essential transport, the emergency services and energy industry – will allow people identified as contacts by NHS Test and Trace or the app to carry on working if their failure to do so would have a “major detrimental impact” or risk national security.

The policy only applies to named workers who are fully vaccinated and it is not a “blanket exemption” for all employees in a sector – for instance, while railway signal operators on whom the network depends may be given an exemption, individual train drivers are unlikely to be.

The Scottish Government also plans a scheme to exempt from self-isolation critical workers in fields including health and social care, and will announce the details on Friday.

Hannah Essex, from the British Chambers of Commerce, said: “While the announcement of a process which may exempt select critical workers from self-isolation in England will be a relief to some businesses, it will leave many more still facing critical staff shortages and lost revenue as the number of people being asked to isolate remains high.”

Confederation of British Industry director general Tony Danker said: “The current approach to self-isolation is closing down the economy rather than opening it up.”

Businesses have already exhausted contingency plans to get in extra staff and are “at risk of grinding to a halt in the next few weeks”, he said.

Meanwhile, officials announced a further 39,906 lab-confirmed Covid-19 cases in the UK and that an additional 84 people had died within 28 days of testing positive for Covid-19.

Professor Stephen Reicher, a member of the Scientific Pandemic Insights Group on Behaviours (Spi-B) -, which advises the Government, told BBC’s Newsnight: “I wish we’d stop talking about the ‘pingdemic’ because the ‘pings’ actually tell you that there’s a problem so you can do something about it.

“The problem is people being in contact with those who are infected because so many people are infected.

“The issue in the end is: what are we going to do about that level of infections?”