He also said that while the initial alert system was completely anonymous, the NHS may later ask users to volunteer location data to help the health service track virus hotspots. Once downloaded, the app would use phones’ Bluetooth connections to log which ones came into close contact with each other.

The Telegraph understands that when a user comes down with flu-like symptoms, they will be invited to answer questions in the app which will determine whether the symptoms meet a threshold of the likelihood of being Covid-19.

Mr Gould said: “If you become unwell with symptoms of Covid-19, you can choose to allow the app to inform the NHS which, subject to sophisticated risk analysis, will trigger an anonymous alert to those other app users with whom you came into significant contact over the previous few days.  

“The app will advise you what action to take if you have been close to someone who has become symptomatic – including advising you to self-isolate if necessary.”

It is understood that if someone then tests negative after the alert is sent out, a second will be sent telling people they can stop self-isolating.

NHSX’s decision to trigger the alert before tests confirm if a user has coronavirus comes after a study by Oxford University last week called for earlier app warnings, saying around half of infections happened while people were not showing symptoms.

The study also found that, if 56 percent of people use the contact tracing app, it could effectively halt the virus before a vaccine or cure is found. That means around 30 million people would need to download the app for it to be most effective.

Mr Gould added: “This new app has the potential to contribute towards the country returning to normality – but only if a large proportion of the population installs it.  That means millions of us are going to need to trust the app and follow the advice it provides.” 

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