The NHS Covid app is "pinging" some phones and not others despite them being exposed to the virus in the same location at the same time, a Telegraph investigation has revealed.
Only one of two Apple iPhones received a self-isolate warning after both were carried by a reporter for almost a week and taken to a number of indoor venues.
The findings have prompted a former government adviser for the app to call on ministers to make the self-isolation notifications less stringent for people who have had both vaccine doses.
The investigation comes as the NHS app pinged a record number of people last week, with 530,126 being advised to isolate for up to 10 days. The mass alerts sparked chaos for businesses and public services.
With scrutiny of the app mounting, a number of users also reported inconsistencies with the technology as people pinged after trips to restaurants or pubs found those with whom they had dined had not received alerts.
Why the NHS app pings some phones but not others
This week, The Telegraph checked into a number of restaurants including one in London’s West End, between Monday and Thursday with two separate phones, an iPhone 7 and an iPhone 6S, after setting up the NHS Covid app on both devices.
On Friday afternoon at 1pm, the iPhone 7 was pinged with instructions to isolate until midnight next Sunday July 25. However, no alert was triggered on iPhone 6S.
The app works by using phones’ Bluetooth radio signals, originally designed to connect devices such as wireless headphones, to detect when other phones with the app are nearby.
It then anonymously logs phones that have been within two metres for around 15 minutes and triggers an alert if the owner of a logged phone registers a positive Covid test.
Jon Crowcroft, a professor of computer science at Cambridge University, who developed the UK’s first digital contract tracing app in 2009 in response to swine flu, said Bluetooth technology was not designed for tracking people’s proximity.
He said it had been repurposed for contact tracing to use a number of signals from bouncing Bluetooth off nearby phones to "estimate" how close other people are and for how long.
However, while the UK app was very accurate compared to other countries’ versions, the signal could be affected by outside factors such as whether one person had their phone on a table while another had it in a bag, which would dull its ability to detect other devices.
Number of contact tracing alerts sent in England
Prof Crowcroft said Bluetooth also had varying effectiveness in different models of smartphone, but that tech giants such as Apple and Google had worked to standardise how their handsets reacted to the UK app.
The academic, who advised the Government on the development of the NHS Covid app, urged ministers to change its isolate advice from a "one size fits all" approach to recoginsing that vaccinated people are at less risk of contracting and spreading the virus.
"I would change the advice to the public in the text [of the notification] it gives you," he told The Telegraph. "If the app knows that you have marked yourself as double-dosed it could say you don’t have to isolate or give less strong advice."
The recent surge in notifications has already led many people to delete the app for fear of being pinged.
Sally Hadley, 76, from Hitchin in Hertfordshire, was pinged just before Christmas after going for a pub lunch with her daughter Lucy Emerson, who did not receive a notification.
The retired actress said: "I couldn’t work it out at all. About two weeks ago I deleted it as there are so many people getting pinged. I did feel a bit bad deleting it, but it has all become a bit of a nonsense, frankly."
The Telegraph contacted the Department of Health for a response.