Covid-19 deaths could be twice as high over the winter as they were in the first wave of the pandemic, PM Boris Johnson is expected to warn MPs later.
In a Commons statement he will say there is “no alternative” as he seeks to win support for a planned four-week lockdown in England from Thursday.
But Mr Johnson will explain 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, due in the House of Commons at about 15:30 GMT, Mr Johnson is 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 is 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 is 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.