Covid-19 deaths could be twice as high over the winter as they were in the first wave of the pandemic, PM Boris Johnson has told MPs.
In a Commons statement he said there was "no alternative", as he seeks to win support for a planned four-week lockdown in England from Thursday.
But Mr Johnson explained he was "right to try every possible option" before ordering people to stay at home.
Labour has said it will back the lockdown but criticised the delay.
Mr Johnson announced at a Downing Street news conference on Saturday that strict measures will include closing pubs, restaurants, gyms, non-essential shops and places of worship.
However, Housing Secretary Robert Jenrick confirmed that moving house will still be allowed during the restrictions, adding that removal firms, estate agents and tradespeople can continue to work but must follow Covid safety guidelines.
Chancellor Rishi Sunak told BBC Radio 4's Today programme that the UK-wide furlough scheme, which had been due to end on 31 October, was extended until December "to give businesses that ease at this difficult time".
He said it was his "expectation and hope" that the English lockdown would be "sufficient" to "exit back into the tiered approach" in a month's time.
Business Secretary Alok Sharma defended the government's lockdown decision at the Confederation of British Industry Annual Conference on Monday, saying: "The cost of inaction would be greater than the action."
In his address to MPs in the House of Commons, Mr Johnson was expected to say: "Models of our scientists suggest that unless we act now, we could see deaths over the winter that are twice as bad or more compared with the first wave.
"Faced with these latest figures, there is no alternative but to take further action at a national level."
Mr Johnson was expected to acknowledge that some MPs believe "we should have reached this decision earlier", but will defend his earlier policy of trying to control the virus with "strong local action and strong local leadership".
Prof Andrew Hayward, professor of infectious disease epidemiology at University College London, told BBC Radio 4's Today programme the forecast death toll data provided to the government included a "range of predictions".
"From the multiple models they are all really showing unacceptable levels of death and the fact NHS services will be overwhelmed within weeks," he said.
The prime minister was due to tell MPs that the government will "seek to ease restrictions" on 2 December and return to the current three-tier system.
On Sunday, Cabinet Office minister Michael Gove said the lockdown could be extended if it took longer to bring the transmission rate of the virus down.
Those around the prime minister say he sees a moral and medical responsibility to act, in order to avert disaster.
The thrust of Boris Johnson's argument in the Commons later will keep returning to the data presented to the country on Saturday night: the range of projections, all bleak, about the grim consequences of doing nothing.
But he'll face sharp, and opposing, questions from behind him, on his backbenches, and opposite him, from Labour and others.
Among the swirl of questions there is testing and tracing, the furlough scheme, the prospect of the English lockdown being extended and an exit strategy.
Lives, liberties and livelihoods: the intensely difficult decisions, with a huge amount at stake, are not going away.
Mr Johnson faces a rebellion from several senior Tory MPs, including Sir Graham Brady, chairman of the 1922 Committee of backbenchers, when they vote on the measures on Wednesday.
Mr Brady told BBC Radio 4's The Westminster Hour the "repetitive cycle" of lockdowns was damaging livelihoods, relationships and mental health.
What are the new restrictions in England?
- People will be told to stay at home except for specific reasons
- These include work which cannot be done from home, childcare or education, exercise outdoors, medical reasons, essential shopping, providing care for vulnerable people or for volunteering, and visiting members of your support bubble
- Meeting indoors or in private gardens will not be allowed, but individuals can meet one other person from another household outside in a public place. Children under school age and those dependent on round-the-clock care who are with their parents do not count towards the two-person limit
- Non-essential retail will close, but can remain open for click-and-collect delivery
- Pubs, bars, restaurants will have to close, but can still provide takeaway and delivery, excluding takeaway of alcohol
- Indoor and outdoor leisure facilities, such as gyms and swimming pools, will also close, along with entertainment venues and personal care facilities such as beauty salons
- Places of worship will close, unless they are being used for funerals, to broadcast acts of worship, individual prayer, formal childcare, or essential services such as blood donation or food banks
- Construction sites and manufacturing workplaces can remain open
- Weddings and civil partnership ceremonies will not be able to take place except in exceptional circumstances, and funerals will be limited to a maximum of 30 people
- Children will still be able to move between homes if their parents are separated
- Clinically vulnerable people will be asked to be "especially careful" but people will not be asked to resume shielding
- Overnight stays, staying in a second home, and holidays will not be allowed - including in the UK and abroad - although there are exceptions, such as work trips
- People will be told to avoid all non-essential travel by private or public transport
Former cabinet minister Esther McVey said she would vote against the restrictions, saying "the world cannot be put on hold" with the "disastrous effects" lockdown "brings to our lives".
Ex-party leader Iain Duncan Smith said the lockdown decision was a "body blow" and accused the government of "giving in" to scientific advisers.
The BBC's Political editor Laura Kuenssberg spoke to Tory MPs ahead of the PM's statement, who she says were "angry, not just about the decision, but about how it's been handled, and No 10's attitude to those arguing for a different course".
4. But - while some of the wannabe rebels are well known like Sir Graham Brady, only a dozen or so at the moment are prepared to say publicly right now they will vote against govt plan on Wednesday, others estimate as many as 40 might rebel— Laura Kuenssberg (@bbclaurak) November 2, 2020
5. But Labour will definitely back the plan - so no chance in any case that the lockdown will be blocked, but the level of Tory support does matter, backbenches already pretty hacked off after v bumpy few months— Laura Kuenssberg (@bbclaurak) November 2, 2020
Downing Street has said that MPs will also get to vote on the "way forward" and what should replace the new lockdown regulations when they expire at 00:01 GMT on 2 December.
But the prime minister will have the support of Labour leader Sir Keir Starmer, who said the measures were "necessary" while criticising the government for delaying them and ensuring what would be a longer, harder lockdown.
On Sunday, the UK recorded 23,254 new confirmed cases of coronavirus and 162 deaths within 28 days of a positive test.
Elsewhere in the UK, Scotland's five-level system of Covid restrictions has now come into force.
In Wales, First Minister Mark Drakeford has said the 17-day "firebreak" lockdown will end as planned on 9 November.
Schools in Northern Ireland will reopen on Monday after an extended half-term break, while other restrictions including the closure of pubs, bars and restaurants continue until 13 November.
In other key developments:
- The Queen and the Duke of Edinburgh have together returned to Windsor Castle - where they have a reduced household staff, nicknamed HMS Bubble - ahead of England's second national lockdown. The 94-year-old monarch had spent the weekend visiting the 99-year-old duke in Norfolk
- Leaders from the UK's four nations have said they will "work together" on a "joint approach to the Christmas period" following a meeting of the Westminster government's emergency Cobra committee
- Remembrance Sunday events in England can go ahead despite the lockdown, Downing Street has said. The prime minister's official spokesman said guidance would be given to councils, adding that events should be held outside with social distancing
- All 40 FA Cup round one ties, including those involving non-elite teams, will be played as planned from 6-9 November. But golf courses, driving ranges and tennis courts in England have been told to close until at least 3 December
- A Winter Defence Force of 7,500 military personnel has been put on standby to help the government and councils deal with the second wave of Covid-19, Defence Secretary Ben Wallace has told MPs. He said the Ministry of Defence was currently looking at a number of "significant asks" from local authorities
- THE NAKED SCIENTISTS: Why are people catching coronavirus on purpose?
- BBC IPLAYER EXCLUSIVE: Would you like a sneak peek at the new series of His Dark Materials?
What questions do you have about the latest lockdown developments?
Use this form to ask your question: