Ministers have ruled out extending the list of workers who are exempt from self-isolation rules and warned that the August 16 date for lifting quarantine for double-vaccinated people could be delayed.

George Eustice, the environment secretary, said he recognised the “stress” that staff shortages were causing for businesses after a record 618,903 people were pinged by the NHS Covid-19 tracing app last week.

However, he defended the government’s decision to limit the lifting of self-isolation requirements to food supply workers, saying ministers were keeping “a very close eye” on the number of hospital admissions.

He also suggested that the government could delay the August 16th date when all self-isolation rules are lifted for double-vaccinated people, saying the government had only announced the date to give people “some kind of indication” of when rules might change. He warned that the date could still move “in either direction”.

Supermarket depot workers and food suppliers will no longer have to self-isolate for ten days if they come into contact with a positive case and will be able to take daily tests instead. The government has also published a list of 16 industries where businesses can apply for an exemption from self-isolation if a worker is pinged by the app.

However, businesses have said the relaxation of measures do not go far enough and have called for the August 16 deadline to be brought forward.

Rail workers warned commuters to expect line closures next week because of staff shortages. Port operators said they were baffled why they had not been included in the same category as supermarket depot workers and food manufacturers.

Tim Morris, chief executive of the UK Major Ports Group, said: “Employers now need to go through a laborious process of producing, agreeing and receiving official letters of approval from the government of named individuals. However, it seems that thousands of supermarket staff get waved through without any such process. We simply don’t understand why it’s one rule for one sector and another for the critical pinch points in the economy.”

Eustice was asked this morning if the exemption list would be expanded to include workers from other sectors beside the food supply industry.

He told Sky News: “The reason we’ve made a special exception for food is for very obvious reasons — we need to make sure that we maintain our food supply.”

“When it comes to other sectors, yes, of course the fact that they are also carrying high absence levels is causing some stress for them and making it more difficult.”

“You also have to bear in mind why we’re doing this and we are trying to still just dampen the pace and the velocity at which this infection is spreading because we have to keep a very close eye on those hospitalisations.”

The minister was also asked why the government had announced a firm date of August 16 for lifting self-isolation rules rather than ease measures closer to the time on the basis of infection data.

He said: “I think people want to have a clear idea of where they stand. The reason we have set that date — of course, things could always change in either direction — but the reason we set these dates is to give people some kind of indication of what they can expect.”

Within the food supply industry, there was still confusion this morning about what workers would be exempt.

Richard Harrow, chief executive of the British Frozen Food Federation, said the exemption list was “worse than useless”.

Nick Allen, chief executive of the British Meat Processing Association, said: “We’ve been told there will be more detail coming out on Monday, which I think is disappointing. We’re delighted that the government has recognised there was a problem bordering on a crisis and something needed to be done but there needs to be recognition this is a 24/7 supply chain, and you can’t wait to Monday to understand who’s going to get this and how it’s going to work — I’ve been inundated with questions from members about how it’s going to work that I just can’t answer at the moment.”

Supermarket staff and suppliers will be exempt from quarantine if they have been fully vaccinated.

Workers in 2,000 warehouses and supermarket distribution centres who are identified as contacts of people with covid will be allowed to take daily tests at work instead of self-isolating.

It represents a significant retreat by the government after Boris Johnson said only a “very small” number of people would be allowed to use testing to avoid quarantine. Up to 10,000 staff are expected to qualify for the scheme.