Boris Johnson will set out emergency measures to control the spread of coronavirus amid concerns the NHS risks being overwhelmed.
The Prime Minister will make a televised address at 8pm on Monday after coming under intense pressure to announce a new national lockdown.
Parliament will be recalled on Wednesday and Downing Street said that the move was in response to the “rapidly escalating” numbers of infections following the emergence of the new variant.
The UK’s chief medical officers agreed to raise the Covid-19 alert level to five – its highest – as they warned that the NHS could be overwhelmed within 21 days “in several areas” without further action.
A No 10 spokesman said: “The spread of the new variant of Covid-19 has led to rapidly escalating case numbers across the country.
“The Prime Minister is clear that further steps must now be taken to arrest this rise and to protect the NHS and save lives.”
Professor John Edmunds, who works on the Government’s coronavirus response as part of the scientific advisory group for emergencies (Sage), warned that “really major additional measures” are needed immediately, with school closures being the “biggest lever” available.
Earlier, Mr Johnson had said there was “no question” that further action was needed and there would be “tough” weeks ahead.
Setting out the measures to come into force from Tuesday, the First Minister told MSPs in Holyrood: “It is no exaggeration to say that I am more concerned about the situation we face now than I have been at any time since March last year.”
Senior Cabinet minister Michael Gove was understood to have discussed restrictions with the leaders of Scotland, Wales and Northern Ireland in a call ahead of Mr Johnson’s statement.
Welsh education minister Kirsty Williams went on to announce that schools and colleges in Wales will remain closed in Wales until at least January 18 and move to online learning.
After a recommendation from the Joint Biosecurity Centre, the UK’s four chief medical officers said the UK alert level should increase to level five “in light of the most recent data”.
In a statement, they said: “Cases are rising almost everywhere, in much of the country driven by the new more transmissible variant.
“We are not confident that the NHS can handle a further sustained rise in cases and without further action there is a material risk of the NHS in several areas being overwhelmed over the next 21 days.”
The latest public data show a 41% rise in the number of confirmed coronavirus patients in hospital in England between Christmas Day and January 3, figures which have caused alarm in Whitehall and the health service.
While ministers hailed the rollout of the new Oxford-AstraZeneca vaccine, Mr Johnson warned the nation needed to prepare for some “tough” weeks ahead as the jab was extended to the most vulnerable.
With 78% of England’s population already under the toughest current restrictions, ministers have been examining how successful the Tier 4 measures – which came into force for the first time on December 20 – have been.
Sage’s Prof Edmunds, warning that the UK’s coronavirus death toll could exceed 100,000, said “significant extra measures” are needed to prevent the NHS being overwhelmed with Covid-19 patients.
The London School of Hygiene and Tropical Medicine scientist told BBC Radio 4’s PM programme: “Unfortunately we are going to have to take some really major additional measures, I can’t see any other way out of it.
“The biggest lever that has only partly been pulled is school closures. That would have the biggest effect of a single measure and I can see that happening.”
He later added: “What we have to do now, and it’s horrible I know, but we have to take really quite stringent steps right now and as stringent as we can right now.”
Former health secretary Jeremy Hunt earlier called for immediate action to close schools, shut borders and ban household mixing, saying the situation was “off-the-scale worse” than previous winter crises faced by the NHS.
“In the face of exponential growth even waiting an extra day causes many avoidable deaths so these plans must now be urgently accelerated,” he said.
Senior Tory Neil O’Brien said procedures “need to toughen up at the border” in order to prevent cases being imported – a particular concern given the potential for new variants such as the one in South Africa.
Labour leader Sir Keir Starmer said he hoped the Prime Minister would respond to the “clear calls for tough national restrictions” and also include the closure of England’s schools.
He told the PA news agency new measures were needed to “get the virus under control, protect the NHS and create the space for the vaccine to be rolled out as quickly as humanly possible”.
Sir Keir said the Government should aim to vaccinate two million people a week this month, rising to four million in February.
Ministers hope the Oxford vaccine – which is easier to distribute than the other approved jab from Pfizer/BioNTech – could provide a route out of the coronavirus crisis, but it could be months before sufficient numbers have received their first shot.
On Monday, retired maintenance manager Brian Pinker, 82, became the first person to receive the jab outside clinical trials.
Ministers have said the NHS has the capacity to deliver two million doses a week but supplies are limited.
While some 530,000 doses are to be available from Monday, the Department of Health and Social Care (DHSC) said tens of millions more are to be delivered in the coming weeks and months once batches have been quality checked.
“It’s not so much a manufacturing issue although that’s part of it.
“Each batch needs to be properly approved and quality controlled.”
Mr Johnson has so far resisted calls to close schools.
Education unions have urged the Government to “pause” a return to the classroom until the safety of staff and pupils can be guaranteed.
In a joint statement, the GMB, NAHT, NASUWT, NEU, Unison and Unite unions said there is a “serious risk” of staff falling ill while the rate of infection is so high.
“Bringing all pupils back into classrooms while the rate of infection is so high is exposing education sector workers to serious risk of ill-health and could fuel the pandemic,” the unions said.
However, Mr Johnson – who has made keeping schools open a top priority – said “the risk to kids is very, very small” and “the risk to teachers is no greater than it is to anyone else”.