The end of England's lockdown will not happen with a "big bang" but will instead be a "gradual unwrapping", Boris Johnson has told MPs.
The prime minister made the comments in the Commons ahead of a retrospective vote later on the lockdown measures.
He said the legislation runs until 31 March to allow a "controlled" easing of restrictions back into local tiers.
Labour leader Sir Keir Starmer said the government's decisions "have led us to the position we're now in".
Health Secretary Matt Hancock said there were now 30,074 patients with coronavirus in UK hospitals.
All of the UK is now under strict virus curbs, with Wales, Northern Ireland and most of Scotland also in lockdown.
It came as the UK reported a further 1,041 people have died with coronavirus, the highest daily death toll since April.
In a statement to the Commons, Mr Johnson said the new variant had "led to more cases than we've seen ever before" and that this had left the government with "no choice but to return to national lockdown".
He said the legislation ran until the end of March "not because we expect the full national lockdown to continue until then, but to allow a steady, controlled and evidence-led move down through the tiers on a regional basis".
He said this would happen "brick-by-brick... without risking the hard-won gains that protections have given us".
But in response to MPs' questions, he said there was a "cautious presumption" that restrictions could start being eased from mid-February.
"And as was the case last spring, our emergence from the lockdown cocoon will be not a big bang but a gradual unwrapping," he added.
Under the measures, which came into force legally on Wednesday, people in England will only be able to go out for essential reasons, for exercise outdoors only once a day, and outdoor sports venues must close.
Police have the powers to enforce the new restrictions with a £200 fine for each breach, doubling on every offence up to a maximum of £6,400 - and a £10,000 penalty for mass gatherings.
Officers in London arrested at least a dozen people in Parliament Square after a protest against the new measures on Wednesday.
The need to debate and vote on the restrictions means the Commons has been recalled from its Christmas break for the second time - the first being for the post-Brexit trade deal with the EU.
With Sir Keir saying Labour will support the motion, the measures are expected to pass with ease.
The restrictions will be kept under "continuous review", Mr Johnson added, with a statutory requirement to reconsider them every two weeks.
Addressing the closure of schools, the PM said "we did everything in our power to keep them open as long as possible" and that was why schools were the "very last thing to close".
They would be the "very first thing to reopen" after lockdown - that could be after the February half term - but "we must be very cautious" about the timetable, he said.
Meanwhile, Education Secretary Gavin Williamson told the Commons that GCSEs, A-level and AS-level exams would be cancelled this year in England, replaced by a form of teacher-assessed grades.
"This year, we're going to put our trust in teachers, rather than algorithms," he said, referencing controversy over the way exam grades were awarded to some students last year.
All national curriculum tests for primary school children, often known as Sats, are now cancelled, Mr Williamson confirmed.
He said every school will be expected to provide between three and five hours of virtual teaching each day and that 750,000 laptop and tablet devices will have been distributed by the end of next week.
The prime minister wasted no time in emphasising the "fundamental difference" between this and previous lockdowns.
To keep opposition from his own MPs at bay he needs to demonstrate that the government's aim to vaccinate the most at-risk groups by mid-February is viable.
He is also under pressure to give a sense of how quickly restrictions might be lifted after that.
The course of the pandemic has changed swiftly at times, though, and may do so again, so it's unlikely we'll get any firm new timelines from Boris Johnson today.
Most Conservative backbenchers seem resigned to the need for this new national lockdown and agree the prime minister had "no choice" but to act.
But MPs on all sides are impatient to hear how soon things may start returning to something like life as normal at last.
Mr Johnson said unlike in March last year, during the first lockdown, vaccines offered "the means of our escape".
But he said there was now a race to vaccinate vulnerable people quickly, with the government setting a target of immunising the four most vulnerable groups - some 13 million people - by mid-February.
"After the marathon of last year, we are indeed now in a sprint, a race to vaccinate the vulnerable faster than the virus can reach them," Mr Johnson said.
"Every needle in every arm makes a difference."
Earlier, Covid vaccine deployment minister Nadhim Zahawi said he was "confident" the government would meet its "ambitious" target, adding that community pharmacies would be brought in to assist the vaccination programme.
He told BBC Radio 4's Today programme that new daily vaccination figures for the UK - which will be released for the first time on Monday - will show there has been a "significant increase" in the number of people who have received the jab.
On Tuesday, Mr Johnson said 1.3 million people in the UK had been vaccinated so far.
Mr Zahawi also said nursery schools presented "very little risk", are Covid-safe and he defended the decision to keep them open during England's lockdown.
At-a-glance: New rules in England
- People cannot leave their homes except for certain reasons, like the first lockdown last March
- These include essential medical needs, food shopping, exercise and work for those who cannot do so from home
- All schools and colleges will close to most pupils from Tuesday with remote learning until February half-term
- Early years settings such as nurseries will stay open
- End-of-year exams will not take place this summer as normal
- Elsewhere, university students should not return to campuses and will be taught online
- Restaurants can continue to offer food delivery, but takeaway alcohol will be banned
- Outdoor sports venues - such as golf courses, tennis courts and outside gyms - must close
- But outdoor playgrounds will remain open
- Amateur team sports are not allowed, but elite sport such as Premier League football can continue
Responding to the prime minister's statement, Labour leader Sir Keir Starmer said his party will support the new restrictions and urged people to comply with them.
"The virus is out of control, over a million people in England now have Covid, the number of hospital admissions is rising, tragically so are the numbers of people dying," he said.
"It's only the early days of January and the NHS is under huge strain. In those circumstances, tougher restrictions are necessary."
But he added "this is not just bad luck, it's not inevitable, it follows a pattern" of the government being slow to respond.
"These are the decisions that have led us to the position we're now in - and the vaccine is now the only way out and we must all support the national effort to get it rolled out as quickly as possible."
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