The NHS Covid app is to be made more “risk appropriate” amid fears that the public is deleting it from their smartphones to avoid being forced into home isolation, a leading public health chief has revealed.

Jenny Harries, the head of the new UK Health Security Agency, signalled the move after new figures showed that the number of people “pinged” by the smartphone app had soared by more than 60% in a week.

As the new Delta variant of the virus ripped through the country, a total of 356,036 alerts were sent to users of the app in England in the week to June 30, telling them they had been in close contact with someone who had tested positive for coronavirus.

The number rose from 219,391 the previous week – a jump of 62% – and is the highest weekly figure since data was first published in January.

There are fears that millions of people could be “pinged” and ordered to self-isolate by Test and Trace, with infections expected to hit around 100,000 cases a day in the summer.

Harries admitted to MPs on Thursday that the public were choosing not to use the app.

She told the Commons Public Accounts Committee that work was underway to “tune” the app to take into account people with double vaccinations, adding the public needed to be reassured it existed “for a purpose, not for annoyance”.

“We have a piece of work ongoing at the moment because it is entirely possible to tune the app to ensure that it is appropriate to the risk,” Harries told MPs.

“When the app came into action, we know it has been hugely successful but it has been utilised in a world where we did not have vaccinations.

“So working through what a vaccinated population using the app means is something that we are actively doing at the moment.”

Most of England’s remaining lockdown restrictions are set to end on July 19, but changes to self-isolation rules will not come in for another four weeks.

Health secretary Sajid Javid has announced that from August 16, those who are double-jabbed will be exempt from the need to quarantine themselves at home if they come into contact with a person with Covid.

But the delay has been greeted with fury by businesses and others who believe that fully vaccinated people should be freed from the self-isolation rule once ‘Freedom Day’ itself starts later this month.

Earlier, Rishi Sunak had acknowledged public “frustration” and said that Javid is looking at an “appropriate, balanced and proportionate” approach for the app.

He told Sky News: “I know most people’s concerns rest with how the app is working, and the health secretary is aware of that.

“The app counts for the majority of the people who need to isolate, I understand, on the numbers, and he’s looking at what the most appropriate, balanced and proportionate approach to isolation is in these circumstances.”

Asked by MPs about reports that some people were deleting the app, Harries said: “I am aware that people are choosing not to use the app.”

She stressed it had resulted in 500,000 cases of the disease being detected and given the current third wave said it was a valuable tool in keeping the virus controlled.

“We are seeing a rise in cases so this is not an inconvenience, it’s actually to alert people to the fact that they have been in close contact and that they may be at risk of being infected themselves and passing that infection on to other people,” she said.

Boris Johnson urged the public not to delete the app from their phones, saying “Obviously, I rely on the NHS app as many people do. I hope that it has been useful.

“But the general system, whatever its frustrations – and again, I repeat, I know how difficult it has been for people and I’m sorry for the hassle that people experience as a result of this – it is coming to an end.

“But what we want to do is use this next few weeks just to do more vaccinations until we’re able to move from quarantining, from isolation, to a test and release system.”

Keir Starmer warned on Wednesday that the plans to end restrictions could lead to a “summer of chaos and confusion”, adding many firms fear “carnage” with the loss of staff to isolation, and people having to cancel social plans.

Ahead of the August 16 date, Labour wants a test-and-release system in place to allow double-jabbed people to avoid the need to isolate.

Separately during her evidence to the MPs, Harries confirmed that free lateral flow tests – offered universally under a “get tested” scheme – are expected to end within weeks.

She said school pupils would continue to be offered two tests a week “for the rest of the rest of September and then to be reviewed”.

“And for the general public at the moment it will be possible under the universal offer to have testing until the end of August.”

Harries stressed “nobody has discussed charging” for the tests. However she added: “There is no cliff edge, but there is a plan to be developed going forward based on the epidemiology.”