Boris Johnson is insisting the forthcoming lockdown in England is only planned to last for a month and he will "seek to ease restrictions" on December 2.
He's hoping to ease unrest among his own backbenchers amid growing concern that the month-long lockdown could be extended further toward Christmas after Michael Gove said December 2 was merely a "review" date.
The prime minister will give a lengthy statement to the Commons at 3:30 on Monday explaining why he feels it is necessary to close close pubs, bars, restaurants, gyms and non-essential from this Thursday for a month.
MPs across the Commons, including many Tories, are angry with Mr Johnson, not only about the lockdown, but about the way it was announced following weeks of insistence that more national measures were not necessary, despite advice on the contrary from his own scientific advisers.
Video report by ITV News Political Correspondent Romilly Weeks
Mr Johnson, who first said a regional approach to lockdowns was best, will MPs warn MPs that coronavirus deaths will be "twice as bad or more" compared with the first wave, "unless we act now".
But already a number of Tory MPs have said they will reject the lockdown on Wednesday, when it is put to a vote, with former Cabinet Minister Esther McVey saying she will definitely vote against it and senior Tory Graham Brady saying he probably will.
Discontent among Tories began to grow after Cabinet Office minister Mr Gove said on Sunday the government could extend the national lockdown if data shows the infection rate has not fallen enough.
And Labour - who had been calling for a national lockdown for weeks - is not only criticising the PM for being "slow" to act but also for not taking the forthcoming lockdown "as seriously as he should be".
Shadow Health Secretary Jon Ashworth attacked Mr Johnson for pulling out of an appearance at the Confederation of British Industry Conference, which is happening on Monday.
Mr Ashworth told ITV News: "This prime minister should be rolling up his sleeves and doing everything he can to protect jobs and livelihoods".
Mr Gove told the Sophy Ridge on Sunday programme it would be "foolish" to predict what would happen with the pandemic over the next four weeks.
He added that the government "hopes" that the tough new restrictions "will have significantly reduced the reinfection rate" by December 2.
However, if the R rate (the rate of infection) has not fallen enough “We will always take a decision on the evidence” and “act in the national interest”.
Under the new lockdown, people will be allowed to exercise and socialise in public spaces outside with their household or one other person, but not indoors or in private gardens, and will be able to travel to work if they cannot work from home.
Yet unlike in the lockdown during the first wave of the pandemic, schools, colleges and nurseries will remain open.
Furlough payments at 80% have extended for the duration of the new restrictions.
The hope is that Covid-19 cases will drop low enough to keep on top of outbreaks at a regional level.
But the prime minister has appeared to slap down those comments.
Mr Johnson is expected to update MPs on the latest lockdown rules on Monday, ahead of a vote on Wednesday.
How likely is it the lockdown will last longer than four weeks?
It is thought the PM will tell the Commons: "On December 2, we will seek to ease restrictions, going back into the tiered system on a local and regional basis according to the latest data and trends."
He will also defend his approach, saying: "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. “I know some in the House believe we should have reached this decision earlier, but I believe it was right to try every possible option to get this virus under control at a local level.”
Political Correspondent Paul Brand said the PM's move is in a bid to appease both sides of his party - those who feel a lockdown is needed and those who are against it.
However, speaking to ITV News on Sunday, a member of the government's Scientific Advisory Group for Emergencies (Sage), Professor John Edmunds also suggested the restrictions could be extended beyond December 2 if cases had not fallen enough.
"We are not in a good position to predict, because we haven't done this before," he said.
"But I hope we'll be able to reduce the incidences and get it in a much better position, but I think we'll have to see how we go and revise and look at the end of November to see where we are."
Prof Edmunds continued that we would not see such a rapid reduction in cases as when the UK locked down in the spring due to measures being slightly looser, for example schools and universities will remain open.
He also added that restrictions will need to stay in place once the lockdown is lifted in order to stop cases surging again.
Following his announcement on Saturday, Mr Johnson is facing criticism that his delay in imposing restrictions will have a “very real” human cost.It emerged earlier in October that government scientific advisers called for a short lockdown in September, prompting Labour leader Sir Keir Starmer to urge Mr Johnson to impose such restrictions.
Speaking to ITV News on Sunday, Sir Keir said: "The prime minister should have listened to what I said and acted on it three weeks ago.
"The delay has cost the economy, because lockdown will be longer now, it's cost the opportunity to take the advantage of half-term, and there's a very human cost because back in September when Sage advised a circuit-break, the daily death rate was 11, yesterday when the prime minister finally acted on that advice it was 326, that is the price you pay for delay."
Sir Keir added Labour would vote for the new measures.
The Labour leader continued that he "hoped" the restrictions would be lifted on December 2, but cautioned: "That will only happen if the government takes the opportunity to fix test, trace and isolate.
"We've been promised a world-beating system.
"We don't need that, we need an effective system."
He continued that if NHS Test and Trace was not improved, "we will not get this virus under control".
The most recent figures from Test and Trace show that just 60.3% of close contacts of people who tested positive for coronavirus were reached through the system.
The week before, that ending October 14, the figure was just 60%, the lowest weekly percentage ever recorded.
The British Medical Association (BMA) also said it was “regrettable that warnings from Sage were not actioned as long ago as September 21”.
The BMA was referring to the advice from Sage in mid-September that a two-week "circuit breaker" lockdown should be brought in to halt spiralling coronavirus cases.
When questioned by ITV News why the government had not brought in a lockdown earlier, Mr Gove replied that there is "no perfect time to know when to act".
"The balance that we've always sought to strike is to make sure as much of the economic life of the nation can continue while also protecting the NHS," the Cabinet Office minister said.
He continued that the "malignancy" and "spread" of the virus in the last two weeks was greater than predicted, which is why the government had now decided to bring in the national measures.
The Covid lockdown announced by Mr Johnson only applies to England since health is devolved.
Wales is currently under a "fire-breaker" lockdown until November 9, similar restrictions will end in Northern Ireland on November 13 and Scotland is introducing a new five-tier system from Monday.