Up to 4.5 million people deemed to be at risk of serious illness from Covid-19 will be asked to stay at home or given tailored advice on protecting themselves if cases of the virus return to dangerous levels, The Telegraph can reveal.
Letters containing specific advice for the recipient will be targeted at individuals identified using a new "risk model" based on factors such as their underlying health conditions, age, sex and weight.
Initially, the shielding programme is due to operate in local areas experiencing sufficiently severe levels of infections, but a Whitehall source said: "If the rate is so concerning across the whole of England, we are prepared to do it on a blanket basis." (Use the graphic below to find out about coronavirus cases in your local area)
It comes amid rising concern that Covid-19 is increasingly circulating among the elderly, as The Telegraph revealed that the virus has begun spreading through care homes and infecting vulnerable patients again.
In a nationwide alert to care providers and councils on Friday night, the Department of Health urged bosses to "take necessary action to prevent and limit outbreaks".
The disclosure comes as former chief scientific adviser Sir Mark Walport warned on Saturday that the UK was "on the edge of losing control" of the virus and urged people to continue working from home if they could.
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There were 3,497 new coronavirus cases across the country on Saturday â€“ the second consecutive day on which numbers rose by more than 3,000. But there was also good news as scientists working on the Oxford vaccine resumed trials after they had been momentarily paused last week.
Boris Johnson has shelved plans to order millions of pensioners to stay at home simply on the basis of their age.
Officials had been drawing up the option of a "differential" shielding scheme, which would have involved asking those aged over a certain age to self-isolate so that Covid-19 spread largely only among the "non-vulnerable population".
Instead, the Prime Minister has given the green light to a scheme to shield those identified by a "stratified risk model" overseen by Chris Whitty, the chief medical officer. It would take a significantly more targeted approach than the shielding programme rolled out during the UK's first local lockdown, in Leicester.
Tailored letters from Matt Hancock, the Health Secretary (watch Mr Hancock update MPs on coronavirus in the video below), and Robert Jenrick, the Communities Secretary, will outline people's "personal risk, and the risks of different activities, in order to help them manage individually", a source said.
The plans include giving additional funding to councils in each affected area to enable local officials to check on people who are shielding and deliver food and medicine where necessary.
Ministers have decided against another national effort to deliver food parcels to all shielded people on the basis that many who required that help during the first lockdown are now able to receive supermarket deliveries after companies significantly increased their capacity.
Last month ministers faced a backlash against leaked proposals, reported by The Telegraph, to ask pensioners to shield on the basis of their age while the rest of the population remained free to move around.
But Mr Johnson is understood to have shelved the idea over concerns about its practicalities, including the difficulty of "rolling back" the scheme once the virus began spreading more freely among the younger population (the graphic below shows the increasing spread of the virus among younger age groups).
Earlier this year, more than two million people were asked to shield in their homes until August 1.
Under the plan signed off by Mr Johnson, up to 4.5 million people will be targeted under the new enhanced shielding programme, based on a combination of factors including age and particular risk factors that have been identified since March.
Government research has found that, among people already diagnosed with Covid-19, those aged 80 or older were seventy times more likely to die than those under 40. Officials have also said that people with diabetes, heart disease and dementia all appear to be at higher risk of death.
A Department of Health spokesman said: "We keep all aspects of our response to the pandemic under review and in line with the advice of our scientific and medical experts.
"Shielding for the clinically extremely vulnerable has been paused since the start of August in most of the country while average rates of coronavirus remain low. Shielding is still advised in specific areas of the country where prevalence of the virus is higher."
Meanwhile, a poll by ORB International found that many young people are failing to follow Government guidelines on social distancing, with one in five (22 per cent) of 18 to 24-year-olds admitting that they are failing to distance from friends and family outside their households.
Overall, 11 per cent of people said they were not, for the most part, still following the rules. One in five (21 per cent) of respondents agreed that gathering with friends and family outside their households was no longer a risk to their health, rising to one in three of those aged between 18 and 34.
The findings come after Mr Hancock urged young people not to "kill your gran" by failing to observe the rules amid a significant increase in infections.