Now London’s deputy mayor for fire has suggested it will not be ready for the capital’s dense housing until later in the year at the earliest, because of the risk of the wrong people being ordered to isolate themselves.
“You might get inaccurate readings from people who haven’t actually been in contact with people.”
Ms Twycross added: “It is more important that we get the app right for when we need it in the autumn, than rush it out before we’re ready.”
Arguing the physical contacting of people suspected to have been in contact with coronavirus carriers was more important, she said: “The test-and-trace and isolate programme is not reliant on the app – nor should it be
“We can’t have a system which it is entirely reliant on people having the right technology and downloading an app to protect the public.”
It is intended to work by using a phone's Bluetooth connection to look for other phones that have it installed.
It logs the phones – and people – that a person has been in contact with, and anonymously alerts them if anyone in that network starts showing the symptoms of Covid-19.
However, the delay and uncertainty has forced the government to deny that ministers are cooling on the whole idea, in favour of relying on making visits and phone calls to people needing to stay home.