The public should download a new contact-tracing app as part of the drive to save lives from Covid-19 and protect the NHS, health bosses have urged.
England's deputy chief medical officer, Professor Jonathan Van-Tam, said there was "fairly significant optimism" that people would use the app – which could be launched within weeks – as part of measures to ease the lockdown.
The app – which asks for the first part of a user's postcode – allows people to alert the NHS if they have symptoms and book a Covid-19 test.
The app tells then tells them to self-isolate while they have their test and await the results.
It also alerts anyone who has been in close proximity with the first user, telling them they should isolate for 14 days.
If the first person's test comes back negative, the app tells both them and their contacts to come out of isolation and carry on as normal.
But if their test is positive, everyone carries on isolating – with the contacts told to book their own Covid-19 test if they themselves develop symptoms.
Prof Van-Tam said: "In relation to the app, clearly the uptake is going to be very important, and so is the compliance with the app and regular using it over a sustained period of time.
"Those are unknowns at the moment but the market research we've done indicates, I think, for the UK, that there's a fairly significant optimism that people will engage with something that clearly is about protecting the NHS."
Matthew Gould, chief executive officer of NHSX – the digital arm of the health service, admitted the app was not a "silver bullet" solution, but said: "The level of impact of the app depends on the level of uptake.
"We are going to mount a really serious campaign to make sure that people know that if they do want to carry on saving lives, protect the NHS, and get the country back on its feet then downloading the app is one way they can do that.
"The earlier we can identify and isolate those who have been in contact with cases of Covid-19, the more confidence we can have in releasing restrictions."
Mr Gould later told MPs that even if only 20% of people downloaded the app, it would lead to important insights about the spread of Covid-19.
But he added: "If we can get to higher levels – 40, 50% or above – the app can really make a big difference in identifying those who have been in touch with suspected cases of Covid-19 and making sure we can identify and isolate those people earlier and faster and more effectively."
Mr Gould said he wanted to reassure the public that privacy is "right at the heart" of the app, adding: "We're not saying people have to give personal information in order to use the app.
"The one thing we're asking for is the first half of the postcode to build up hotspots and manage the crisis.
"It (the app) doesn't know me as 'M Gould' – it knows me as a number."
For anyone who does not have a smartphone, around 18,000 contact tracers will be on hand to help trace the contacts of people who test positive for Covid-19.
Taken together, the aim is to cut off routes of transmission for the virus, therefore minimising outbreaks and preventing a second peak of Covid-19 in the UK.
Stressing the app's importance, Prof Van-Tam said it was needed to give "room for manoeuvre in terms of other social distancing easements" that can be made in the future.
And he said the use of Bluetooth technology would allow experts to "be able to learn, for example, about whether 30 minutes' exposure at close quarters to a case on day one of their symptoms is equivalent to two hours' exposure on one-and-a-half days before their symptoms began."
Addressing security, Professor Sir Jonathan Montgomery, of University College London – who is head of an ethics board overseeing the app's development, said that even if hackers did manage to get into any database, they would "not find any identifiable information about the users".
He added: "Each phone has a randomly allocated identifier, a pseudonym."
Meanwhile, a senior Government source said that where outbreaks are detected, officials are examining ways of imposing targeted regional lockdowns to prevent the virus spreading out of control.
"What potentially could happen is that if you were to see spikes in the disease in specific regions of the country you could reimpose some of the social distancing restrictions on a regional basis," they said.
The time to move onto the second phase will be once we have met the 5 tests.
— UK Prime Minister (@10DowningStreet) May 4, 2020
The Government has to review the current lockdown measures by May 7 and, while the current restrictions are not expected to be lifted, Prime Minister Boris Johnson is expected to set out a "roadmap" on Sunday setting out the next steps.
Scotland's First Minister Nicola Sturgeon, who has been closely involved in Cobra meetings determining the strategy, said it was "very likely" the lockdown would continue as the infection rate was too high to make "any meaningful change".
In other developments:
– Defence Secretary Ben Wallace said the Government has been "trying to source as many masks as possible" in case guidance for their use by the public changes
– Business Secretary Alok Sharma confirmed that 601 people have now taken part in the Oxford University clinical trial to try to find a vaccine
– Former Government chief scientific adviser Sir David King hosted a meeting of experts on Monday in response to concerns over the "lack of transparency" coming from the Sage group of Government advisers.
– Mr Johnson is expected to face Labour leader Sir Keir Starmer at Prime Minister's Questions on Wednesday, having missed previous encounters due to his battle with coronavirus and the birth of his son, Wilfred.
Meanwhile, new guidance is being prepared for businesses on how they should operate when the lockdown is eased, including reduced hot-desking and the closure of office lifts and canteens.
Earlier, officials announced that the Nightingale hospital in London's ExCeL centre was being put on standby, adding that it was a "positive thing" due to a slowdown in coronavirus cases.