The app was originally set for release in May but was hit by development problems, forcing the government to U-turn and use Apple-Google software instead.
The UK government officially launched its troubled track and trace app on Thursday, four months behind schedule and as the country faces its highest daily COVID-19 case since the peak of the pandemic.
Urging people in England and Wales to download the app, Health Secretary Matt Hancock said the UK was "at a tipping point" in its efforts to prevent a second wave of coronavirus this winter.
The launch comes as the UK saw 6,178 new positive cases on Wednesday, the highest daily infection rate since May 1.
In a statement, he said: "With infection rates rising we must use every tool at our disposal to prevent transmission, including the latest technology.
"Today’s launch marks an important step forward in our fight against this invisible killer and I urge everyone who can to download and use the app to protect themselves and their loved ones."
Originally, the UK government advised a track and trace app would be ready by May but the project was beset with problems and delays.
In an embarrassing U-turn, the government turned away from home-grown technology, which was deemed less effective, in favour of Apple and Google software in June.
"We have worked extensively with tech companies, international partners, and privacy and medical experts – and learned from the trials – to develop an app that is secure, simple to use and will help keep our country safe," said Hancock.
The new COVID-19 NHS app, which is based on Bluetooth technology, will alert users if they have spent 15 minutes or more within two metres of another user who subsequently tested positive for the virus.
It will also make it easier for customers to provide pubs and restaurants with their contact details - now a legal obligation - thanks to a QR code reader.
"It is possible to significantly reduce the number of new coronavirus cases, hospitalisations and deaths if just 15 per cent of the population downloads the app and follows isolation advice," Christopher Fraser, professor at the University of Oxford's Big Date Unit, said.
The app is exclusively for use for over 16-year-olds in England and Wales.
Northern Ireland and Scotland already launched their own track and trace apps at the end of July and the start of September respectively.
The UK has recorded the highest number of deaths in Europe, according to official figures, with 41,862 fatalities up to Wednesday.
On Tuesday, Boris Johnson called for a "spirit of togetherness" in a live address to the nation as he tightened restrictions to tackle the surge in new cases, including a 10 pm curfew for pubs and restaurants, stricter rules on masks and encouraging remote working from home.
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