Health Secretary Matt Hancock defended the decision to abandon developing the NHSX app in favour of joining forces with the Apple and Google project.

At the Downing Street press conference he said testing on the Isle of Wight uncovered a "technical barrier".

"We found that our app works well on Android devices but Apple software prevents iPhones being used effectively for contact tracing unless you are using Apple's own technology," he said.

He said that the NHSX app was better at measuring distance than the Google/Apple model.

"As it stands, our app won't work because Apple won't change their system, but it can measure distance.

"And their app can't measure distance well enough to a standard that we are satisfied with."

Health Secretary Matt Hancock said: "We have agreed to join forces with Google and Apple to bring the best bits of both systems together.

"We will share our algorithm and the work that we have done on distance calculation and combine that with their work to deliver a new solution."

That would "bring together some of the best minds to find a solution to this global challenge".

Health Secretary Matt Hancock set out who could be first in line for a coronavirus vaccine if one is proven to be effective.

At the Downing Street press conference he said the joint committee on vaccination and immunisation had published interim advice.

"They recommend priority vaccination for two groups: frontline health and social care workers and those at increased risk of serious disease and death from coronavirus."

That would include the over-50s and those with heart and kidney disease.

"As we learn more about the virus we will continue to take into account which groups may be particularly vulnerable, including, for example, those from ethnic minority backgrounds so that we can protect the most at risk first, should a vaccine become available, and get this country back on our feet as soon as we possibly can."

Health Secretary Matt Hancock could not say when the contact tracing app would be ready for roll-out.

He told the Downing Street briefing: "We're working on it. We're not going to put a date on it I'm afraid because I'm absolutely determined that, whilst this technology can help, it's got to be working effectively.

"But I am confident we will get there - we will put that cherry on Dido's cake."

NHS Test and Trace programme chief Baroness Harding said none of the apps were reliable enough to work.

At the Downing Street press conference she said: "This is a global challenge. What we have done in really rigorously testing both our own Covid-19 app and the Google/Apple version is demonstrating that none of them are working sufficiently well enough to actually be reliable to determine whether any of us should self-isolate for two weeks.

"That's true across the world."

Having a working app would be a "hugely helpful addition" for people who commute and are forced to mix with strangers, but "good hand hygiene", following social distancing and wearing masks were important steps to take now.

Baroness Harding admitted the programme was "not perfect".

Speaking at the Downing Street briefing, she said more people needed to get tested for any symptoms associated with coronavirus, while 70% of people would not recognise that they had contracted Covid-19.

Lady Harding said: "We have to get better at hunting out the virus.

"Seventy per cent of people or so who have the virus won't show any symptoms or they will have such mild symptoms they might not spot it.

"That's why we are doing targeted testing for people in high-contact professions such as health and social care, but also other roles where we are looking to expand that."

She added: "I'm not pretending it is perfect, of course it isn't - we all have work to do to keep honing it and improving it."

Health Secretary Matt Hancock defended the Government against allegations it had spent too long trying to build its own app instead of using the Google-Apple technology.

He told the Downing Street briefing: "No, actually quite the contrary, I'm from Newmarket, we back both horses.

"We took the decision in May to start building the Google-Apple version as well and then because we built both we could test both.

"And actually the best way to get new technology going is to test different approaches.

"Far better to go with both versions and now we've got problems with both versions but there's parts of each that can come together to build something that's stronger than either version."

Baroness Harding, the executive chair of NHS Test and Trace, added: "The reality is if we had not backed both horses we wouldn't have a way forward.

"Precisely because we've developed our own and developed some really sophisticated distance calculations we think that we can enhance the Google-Apple platform such that it will work."

The Health Secretary said the test and trace system had meant local outbreaks were known about.

Matt Hancock said the manual tracing system had helped control local outbreaks in "parts of Leicester" and Kirklees, West Yorkshire, with mobile testing units sent in to help track down positive cases.

He said: "There are these local outbreaks, we've seen them all the way through.

"And we now have the system in place to be able to spot them the best we can and then take that local action.

"It is natural that as the lockdown lifts slightly, we've always said that the next move is for more localised action in order to tackle outbreaks where we find them."