Professor Paul Hunter, an expert in health protection at the University of East Anglia who has previously advised the World Health Organisation, said the app could be an effective tool in keeping down infection rates if people followed the self-isolate advice.

However, he warned a button that effectively allowed users to reset the self-isolate timer undermined how serious people would take the app’s warnings.

Prof Hunter told The Telegraph: “Anything that undermines the app’s purpose in terms of reducing contacts makes it not fit for purpose.

“And the fact is 80 per cent people ignore it (self -isolation alerts) and have got ways around it to salve their own consciences.”

Sources close to the design of the app confirmed that the ‘delete all data’ feature did not remove any logs of close contacts people had had, which are not stored on the phone but a decentralised server.

The Telegraph understands the button was included on the advice of the UK’s data watchdog, the Information Commissioner’s Office, and that one of the reasons was due to fears that the app’s log of venues people had visited could be used by abusive partners to track their partners.

However, Jon Crowcroft, a Cambridge professor of computing who designed the UK’s first Bluetooth contact-tracing app in 2009 for swine flu, said the NHS should alter the delete data function in its next update so that it did not interfere with self-isolate timers.

He said: “It is a criticism and it would be good for version three to say ‘do you want to delete that?’ Because if you can delete the countdown for quarantine that is a bit undermining of the buy-in, as that is a crucial thing.” The Sunday Telegraph has contacted the Department for Health and Social Care for comment.

A spokesman for the Information Commissioner’s Office said data controls were important to gaining public trust for the app.

The spokesman said: “Providing a high level of transparency and putting data protection at the heart of an app or service can help build people’s trust in the systems involved. If people don’t have trust in how their data is being used and have little control, they’re less likely to use an app or service at all.”

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