Boris Johnson has ordered a new national lockdown which means people will only be able to leave their homes for limited reasons, with measures expected to stay in place until mid-February.
People who are clinically vulnerable and who were previously told to shield should stay at home and only leave for medical appointments and exercise, the Prime Minister said.
Primary and secondary schools will close immediately and move to online learning for all pupils except children of key workers and the most vulnerable, Mr Johnson said in a live televised address.
Mr Johnson's statement came after the chief medical officers for the first time raised the UK to the highest level on the Covid-19 alert system.
They warned the NHS is at risk of being overwhelmed within 21 days "in several areas" without further action.
In an address to the nation, the Prime Minister said the new variant - which is 50% to 70% more transmissible - was spreading in a "frustrating and alarming" manner.
"As I speak to you tonight, our hospitals are under more pressure from Covid than at any time since the start of the pandemic," he said.
The Prime Minister said that in England the number of Covid patients in hospitals has increased by nearly a third in the last week to almost 27,000 - some 40pc higher than the first peak in April.
On December 29 "more than 80,000 people tested positive for Covid across the UK", the number of deaths is up by 20% over the last week "and will sadly rise further", he said.
"With most of the country, or maybe under extreme measures, it's clear that we need to do more together to bring this new variant under control while our vaccines are rolled out.
"In England we must therefore go into a national lockdown which is tough enough to contain this variant."
Mr Johnson said: "I know how tough this is, and I know how frustrated you are and I know you have had more than enough of government guidance about defeating this virus, but now, more than ever, we must pull together."
He said the Government is again instructing people to stay at home.
He said: "You may only leave home for limited reasons permitted in law, such as to shop for essentials, to work if you absolutely cannot work from home, to exercise, to seek medical assistance such as getting a Covid test, or to escape domestic abuse."
The latest figures showed a further 407 people have died within 28 days of testing positive for Covid-19 as of Monday and there were a record 58,784 more lab-confirmed cases of coronavirus in the UK.
Latest figures from Public Health England saw that every area of Norfolk and Waveney saw new record high numbers of confirmed coronavirus cases in the seven days over the Christmas period.
The number of Covid-19 cases in Norfolk was 447.5 per 100,000 people in the seven days up to New Year’s Eve, a 67pc increase from the previous week - the seven days up to Christmas Eve.
Public data showing a 41pc 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 NHS.
While ministers hailed the rollout of the new Oxford-AstraZeneca vaccine, Mr Johnson said that the weeks ahead would be the "hardest yet" but added that he believed the country was entering "the last phase of the struggle".
The clinically extremely vulnerable are advised to begin shielding again, and Mr Johnson said those affected will receive letters "shortly".
Boris Johnson suggested England could "steadily" move out of lockdown from mid-February.
Urging caution about the timetable, he said: "If our understanding of the virus doesn't change dramatically, once again, if the rollout of the vaccine programme continues to be successful, if deaths start to fall as the vaccine takes effect and - critically - if everyone plays their part by following the rules, then I hope we can steadily move out of lockdown, reopening schools after the February half-term and starting cautiously to move regions down the tiers."
Mr Johnson said that the country should "remain cautious about the timetable ahead" due to the time lag in people receiving immunity from vaccines.
He said: "I must emphasise that even if we achieve this goal there remains a time lag of two to three weeks from getting a jab to receiving immunity and there will be a further time lag before the pressure on the NHS is lifted."
While more than 100 primary schools across Norfolk kept their doors closed on the first day back after the Christmas break, scores more did open to all children but now facing closing again after just a day.
Mr Johnson said parents "may reasonably ask why" decisions on schools were not taken "sooner".
"The answer is simply that we've been doing everything in our power to keep schools open because we know how important each day in education is to children's life chances," the Prime Minister added.
"And I want to stress that the problem is not that schools are unsafe for children. Children are still very unlikely to be severely affected by even the new variant of Covid.
"The problem is that schools may nonetheless act as vectors for transmission, causing the virus to spread between households."
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