Boris Johnson is to make a televised address setting out new emergency measures to control the spread of coronavirus in England, Downing Street has said.
A No 10 spokesman said that the move was in response to the “rapidly escalating” numbers of infections following the emergence of the new variant.
The statement, to be made at 8pm on Monday, will be followed by the recall of Parliament on Wednesday so MPs can debate the measures.
“The Prime Minister is clear that further steps must now be taken to arrest this rise and to protect the NHS and save lives.”
The move comes as Nicola Sturgeon announced Scotland will go into lockdown for the rest of January with a legal requirement to stay at home and schools closed to most pupils until February.
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.”
The alert level is based on a recommendation by the Joint Biosecurity Centre to the four chief medical officers.
Earlier, during a visit to a vaccination centre in north London, Mr Johnson acknowledged there was “no question” the further measures would be necessary, as cases continue to surge across the country.
The latest 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.
Former health secretary Jeremy Hunt 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”.
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”.
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 of the Oxford vaccine 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.”
Meanwhile, uncertainty continued around the reopening of England’s primary schools this week.
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”.