People planning to celebrate New Year's Eve have been warned they must leave the parties for later next year as "Covid loves a crowd".
The police have warned flouters could face fines of up to £10,000 with officers to be paying particularly close attention to areas experiencing the biggest infection surges.
It comes as Health Secretary Matt Hancock announced that 20 million people would be plunged into Tier 4 from midnight yesterday.
A total of 44 million people, or 78 percent of the population of England, will be living under the country's toughest restrictions from Thursday as the whole of the UK battles a winter surge.
Large parts of the North and Midlands joined London and the South East, while every area previously in Tier 2 was moved to Tier 3.
Only the Isles of Scilly will be in Tier 1 come Thursday morning, with no areas in Tier 2, so bars and pubs across the land will have to be closed to customers.
Martin Hewitt, chair of the National Police Chiefs’ Council, said he was confident that the majority of people would respect the rules for their area.
“Large gatherings and parties should not be happening,” he added. “We ask anyone thinking of attending or organising one not to."
Professor Stephen Powis, NHS England's national medical director, said staying at home and seeing in 2021 "within the rules" will reduce infections and relieve pressure on hospitals.
He told a Downing Street press conference: "We know it's the end of the year, it's a time where people traditionally want to celebrate.
"But it's absolutely vital that this year, everybody continues to follow the guidance by staying at home and not mixing."
Prof Powis added: "We can all play a part in fighting this terrible virus: stay at home, mark the New Year with just nearest and dearest within the rules.
"This action will reduce infections, relieve pressures on hospitals, and that's how everybody can help to save a life.
"Covid loves a crowd, so please leave the parties for later in the year."
His call for restraint came as deputy chief medical officer Professor Jonathan Van-Tam said the NHS had yet to see the impact of household mixing over Christmas.
He said the situation in the UK is "precarious in many parts already" and urged the public to "play your part from bringing us back from this very dangerous situation".
"It is almost certainly true that the NHS has not yet seen the impact of the infections that will have occurred during mixing on Christmas Day and that is also unfortunately rather sobering," Prof Van-Tam said.
Prime Minister Boris Johnson also urged people to follow the rules on December 31.
He said: "I must ask you to follow the rules where you live tomorrow night and see in the New Year safely at home.
"That means not meeting up with friends or family indoors, unless they're in the same household or support bubble, and avoiding large gatherings of any kind."
The Metropolitan Police issued a warning to potential revellers to "celebrate the New Year in the comfort of their own homes, not the homes of family and friends".
Commander Paul Brogden, who is leading this year's operation, said: "I would urge everyone to exercise caution, listen to government advice and celebrate the New Year in the comfort of their own homes, not the homes of family and friends.
"The public can expect to see officers deployed across the capital, supporting communities and focusing strongly on the few people intent on breaching and ignoring the guidance put in place to keep everyone safe.
"Officers will also be paying attention to parts of London that are experiencing the highest infection rates.
"As we have throughout the year, if we become aware of planned events that breach Covid regulations, we will engage with organisers or venues to make them fully aware of the legal consequences and encourage them to cancel.
"Failure to cancel such events, like unlicensed music events, will likely see the organiser facing a very significant fine.
"Although the New Year is almost upon us, now is not the time for complacency or to meet with others and hold parties of any kind.
"If people insist on gathering and breaching regulations, then officers will attend and encourage people to disperse.
"Where necessary, enforcement action, including fines starting at £100 and working their way up to £10,000, will be considered."
