The NHS coronavirus app may only work when the user’s phone is unlocked and the app is running, it has emerged.
The app, which it’s hoped will be key to the UK’s route out of lockdown, is being trialled in the Isle of Wight from this week.
Matt Hancock said it was the public’s duty to download and use the app.
But as well as privacy concerns about data collected by the app, questions are being raised about whether it will work at all.
The app works by devices ‘pinging’ encrypted signals to each other while users are out and about.
When a user reports coronavirus symptoms, the app looks back through the people they have recently come into contact with and tells them they should self-isolate.
Phone makers Apple and Google offered governments around the world a way to use their operating systems to collect a limited amount of data to help trace and contact people coronavirus sufferers have been in contact with.
Their way would allow devices to send and receive “pings” in the background - saving on battery life and meaning the user didn’t have to keep their phone open all the time.
But the developers of the UK’s app decided the amount of data offered by Apple and Google wasn’t enough - they wanted more so they could mine it for more information to help them plan NHS capacity and track the spread of the disease in more detail.
They say they can only do that by collecting data on a central server - which they say will be anonymised and private, despite asking for the first half of the user’s postcode when they register.
But Apple and Google won’t allow apps which work in this way operate in the background for security and privacy reasons.
It means the iPhone version of the app will most likely only be able to send ‘pings’ when it is running, on the screen and the screen is unlocked.
Android devices will allow apps to broadcast pings in the background, but only for a few minutes.
The app makers argue that the app can still listen for pings when it’s in the background, and that will still allow it to collect some data.
Health Secretary Matt Hancock has insisted people's data will be kept safe.
"I think we can give very significant reassurances on the privacy aspect but what I can tell you is that if you download the app then you are doing your duty and you are helping to save lives, and you're helping to control the spread of the virus, and that's true as of this morning on the Isle of Wight amongst NHS staff, but it'll be true increasingly across the country as we roll it out," he told the BBC.
On suggestions that it could be used for surveillance, he said: "That's completely wrong.
"Firstly because the data is stored on your phone until you need to get in contact with the NHS in order to get a test, and secondly because the purposes of this are purely and simply to control the spread of the virus, which is really important.
"Thirdly because we've all had to give up significant infringements on our liberty, for instance with the social distancing measures and the lockdown, and we want to release those, and this approach will help us to release them... I can reassure you that it's completely untrue."
Dr Ian Levy, technical director at the National Cyber Security Centre (NCSC), has also spoken about the trade-offs.
"The NHSX system as a whole does a good job of balancing the individual privacy needs with the public health needs," he said.
"The privacy and security design is there to support the epidemiological model and the needs of clinicians who are managing the virus in the UK.
"There are balances and trades to be made."
