The NHS's track and trace system will launch tomorrow without a contact-tracing app, despite concerns that manual tracing alone will be too slow to accurately measure another outbreak of the virus.
Baroness Dido Harding, the new boss of the NHS's Test and Trace programme, would not confirm if the app was still on track to be launched in June.
When asked whether she could say when the app would be available, the former TalkTalk boss who is now leading the UK's tracing effort, said: "No, sorry. One of the things I am trying to do with this programme is move away from individual dates."
Contact-tracing apps work using the Bluetooth technology used in smartphones. Once installed, the app records all of the other people a person has been in close contact with during their daily activities by performing a "digital handshake" with other phones it has been in close proximity with for a set period - say 10 minutes.
If a person is later diagnosed with coronavirus, all of their recent close contacts can then be automatically alerted and advised to isolate.
Health Secretary Matt Hancock and Matthew Gould, chief executive of NHSX, the health service's technology arm, had originally suggested the app could be ready by the middle of May.
However, while a trial of the app has been underway on the Isle of Wight since May 5, a wider launch in other parts of the UK has been delayed.
Experts have warned that without a digital contact-tracing app, coronavirus would spread too quickly for manual contact tracing to measure an outbreak.
In April, Professor Christophe Fraser, who is leading the Oxford University team helping develop UK contact-tracing technology, said: "Our analysis suggests that about half of transmissions occur in the early phase of the infection, before you show any symptoms of infection.
"Our mathematical models also highlight that traditional public health contact tracing approaches provide incomplete data and cannot keep up with the pace of this pandemic."
Professor Eivor Oborn, a health technology expert at Warwick Business School, said: "The fact that the trace, track, and isolate system is being launched ahead of the delayed NHS app is problematic. The government had already admitted the app would not be ready to launch on June 1. That implies there have been issues.
"It is imperative the technology actually works. Manual tracing alone is too slow, and if there is a second spike it would be very difficult, if not impossible, to locate contacts fast enough."
At the daily briefing on Wednesday, Matt Hancock said the contact-tracing app being developed by NHSX would be launched "in the weeks ahead... once the system is bedded in".
An NHSX spokesman confirmed the working date for a launch was still some time in the middle of June.
The trial of the new app on the Isle of Wight has run into a number of problems, including getting the system to match iPhones if they are left idle for too long.
The UK is seeking to develop its own system for a contact-tracing app which relies on a centralised NHS database where all of the information is held.
Technology giants Apple and Google have developed their own system, which they claim is more private. It uses a different technology where no centralised database exists and all of the tracing information is held on individual smartphones.
The UK initially rejected that concept because health bosses said the information contained on a central database would be useful in helping inform policy decisions about how to tackle the virus.
The NHS has since been reconsidering Google and Apple's approach.
Michael Veale, a developer at UCL who is working on contact-tracing technology, pointed out that a Swiss system developed using decentralised app technology had already been launched. Although he added the technology was "not necessary" for all contact-tracing.