StopCovid France is designed to prevent a second wave of infections by using smartphone logs to warn users if they have been near someone who later tested positive for the virus.

Health Secretary Matt Hancock had originally said it would be by 1 June, and then later suggested it would be around the middle of next week.

It will also start giving at-risk users a code to enter into a separate website when they book a medical test.

By contrast, Latvia, Italy and Switzerland have released apps based on a "decentralised" technology developed by Apple and Google.

"As of this morning, 600,000 people managed to download the app, so it's a very very good start," he told the TV channel France 2.

The French government had said the app would be released at midday on Tuesday.

Baroness Dido Harding, who heads up the government's Test and Trace programme, said little new about the app when she appeared before the House of Commons' Health and Social Care Committee, beyond saying it was a "high priority" to link it to medical test results

An app, he explained, could still be used to serve users amber warnings - perhaps advising them not to visit an elderly relative or a friend in a vulnerable group - while their contact awaited a result

"The app provides the best early warning system by enabling us to record close physical contact with people we know, but also those we don't know or can't remember," the Oxford Big Data Institute academic told the BBC

Earlier, Northern Ireland's chief scientific advisor told the Stormont Health Committee that he planned to focus on manual contact tracing, saying he thought the app's usefulness had been overstated

The Health Minister Robin Swann added that he had concerns that the app would be unattractive to users because of concerns about it draining battery life, and that people at the end of a phone were already proving effective