Boris Johnson is expected to announce a new national lockdown next week after scientists warned that Covid-19 was spreading faster than their worst predictions.
The Prime Minister spent Friday in crisis meetings with ministers and aides after being told deaths were tracking above the "worst case scenario" that suggested 85,000 in the second wave.
Senior Government sources stressed that no final decision had been made and said the measure would need to be put to the Cabinet before any announcement to the nation.
Mr Johnson is likely to summon ministers from his Cabinet coronavirus subcommittee over the next 48 hours and could hold a full meeting on Sunday if he decides he needs to act as soon as Monday.
The alternative to a national lockdown would be a "fourth tier" of restrictions on top of the existing three-tier system, but Government scientists now believe even Tier 3 is not enough to stop the spread of Covid infections.
Imposing a second national lockdown would be a bitter blow for Mr Johnson, who has insisted for months he did not believe such a move would be necessary.
The Prime Minister described a second lockdown as a "nuclear option" and warned that it would be an economic "disaster". Last month, he told MPs that restrictions would be "completely wrong for the country".
But Government scientific advisers told him that, by October 14, deaths had already reached the daily levels predicted in their worst case scenario planning and would exceed their most pessimistic predictions by the end of the month.
Mr Johnson met Rishi Sunak, the Chancellor, Michael Gove, the Cabinet Office minister, and Matt Hancock, the Health Secretary, in Downing Street on Friday to discuss the next steps.
The ministers discussed closing down all but essential retailers and schools, with universities and nurseries also staying open.
If Mr Johnson decides to announce the measures at a press conference on Monday, they could come into force on Wednesday and last until December 1. Such a move would meet vehement opposition from many Tory MPs, who have insisted that the economy must be protected.
Many retailers make the bulk of their profits in the run-up to Christmas, and a month-long lockdown could be the death knell for businesses that are already struggling to break even.
Another 24,405 people tested positive for coronavirus on Friday, with 274 deaths. The 'R' rate of infection fell week on week from between 1.2 and 1.4 to between 1.1 and 1.3, but that still means infections are spreading exponentially and the Office for National Statistics said cases "continue to rise steeply".
One Cabinet minister said: "When you look at what’s happening in France we might have to adapt the tier system and add extra restrictions. The Cabinet is pretty united on this – we don't want to see a national lockdown because a circuit breaker is not the answer, but we realise we might have to get tougher in the areas where the infections are highest."
Mr Johnson believes the public is ready to accept tougher restrictions. Another Cabinet minister said: "The polling shows that the public are already there – they know that this is going to be difficult for a while and they are supportive of the measures."
To date, 10.6 million people in England have been placed into Tier 3, with another 10.6 million in Scotland, Northern Ireland and Wales living under similar or harsher measures.
The West Midlands and the Tees Valley are expected to be moved up to Tier 3 as early as next week. Officials working for Sadiq Khan, the Mayor of London, believe the capital will be placed into Tier 3 in November, although no formal talks with the Government have yet been held.
If London, the West Midlands and the Tees Valley are put into Tier 3, that would mean 35 million people living under the strictest regulations – more than half the UK population of 66.6 million.
Many Tory MPs have urged Mr Johnson to resist calls to go further with restrictions, but Whitehall sources said there was "concern about the data" in Downing Street.
New documents released by the Scientific Advisory Group for Emergencies (Sage) show that, by October 14, deaths had reached reasonable worst case scenario levels and were likely to exceed them by the end of the month.
Modelling leaked to The Spectator earlier this week showed that the Government expects up to 85,000 deaths in the second wave, but the death toll may now be higher without widespread restrictions, scientists have warned.
Discussions are ongoing about whether the harsher restrictions would be referred to as Tier 4 or "Tier 3 plus".
One senior Government source said: "In Tier 3, everything is up for discussion apart from schools, so 'Tier 4' is an odd concept in that sense."
Dominic Raab, the Foreign Secretary, said the Government was "striving" to avoid blanket restrictions. Asked about the potential for a fourth tier, he said: "We're always ready for further measures that we can take, but I think the most important thing about further measures is that we continue on the track that we're on of targeting the virus."