The expert behind the smartphone app that ministers hope will track and trace potential coronavirus victims – and keep infections down – has warned it will require a 60 per cent take-up to be successful.

Professor Christophe Fraser also agreed that “lives would have been saved” if the government had begun contact tracing earlier, instead of only starting recruitment now.

And he suggested up to 6 million Britons have already contracted the infection, saying: “Somewhere between three and maybe up to 10 per cent of the population.”

The UK is poised to follow in the footsteps of hi-tech tracking trailblazers such as Singapore and Israel by developing software to detect who has been in contact with someone with virus symptoms.

NHSX, the health service’s in-house digital team, is leading the work for health secretary Matt Hancock, who is an enthusiast, with hopes it will launch “within weeks”.

The technology would detect when two smartphone owners are within 2m of each other and check their status against a database of Covid-19 carriers.

If either party triggers an alert, the other would get a warning – and possibly an order to isolate themselves as well.

But in Singapore, the app has been downloaded by less than one in five people – and Prof Fraser, who is leading the Oxford University team developing it, was clear that a high take-up was essential.

“For this intervention alone to stop resurgence of the epidemic, about 60 per cent of the population would have to use the app,” he told the BBC’s Andrew Marr Show.

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“Now that number may be a bit smaller if there are other interventions going on, which we hope there will be; social distancing, large community testing and indeed manual contact tracing.”

The government has been under fire for only starting the recruitment of 18,000 community infection-chasers – having stopped contact tracing in mid-March, as the virus took hold in the UK.

Asked if he thought pursuing this policy earlier could have saved lives, Prof Fraser said: “I think so.

“I worked on the Sars epidemic in 2003 and testing and tracing is really a cornerstone of how you stop a serious infection. And I do think that strategy scaled up is tremendously effective.”

Other experts have cautioned that the app will be unable to achieve what Mr Hancock has claimed and is no substitute for the “shoe leather” of staff carrying out tracing.

Some fear a blizzard of false warnings could actually make things worse and risk undermining public confidence.

The UK app will use the short-range wireless technology Bluetooth, but there are fears it is too unreliable and inaccurate for two moving targets.