Millions of people will no longer be allowed mix with other households at Christmas, with a new Tier 4 level of COVID restrictions now in force.
In a dramatic move, prompted by fears over a new strain of coronavirus, all of those areas previously in Tier 3 in the South East - including London - moved to the new Tier 4 on Sunday.
And, in further stark action across the rest of England, Prime Minister Boris Johnson announced on Saturday that the planned five-day easing of restrictions over the festive period will be limited to a single day.
"Given the early evidence we have on this new variant of the virus, and the potential risk it poses, it is with a heavy heart that I must tell you we cannot continue with Christmas as planned," he said at a Downing Street news conference.
In Wales strict Tier 4 measures also came into force from Sunday - more than a week earlier than planned.
First Minister Mark Drakeford also said households across Wales will now only be able to form a Christmas bubble with one other household on Christmas Day.
Scottish First Minister Nicola Sturgeon told her own news conference on Saturday that the country would be placed into Level 4 rules from Boxing Day, with the easing of restrictions on indoor household mixing now only to apply on Christmas Day.
She also announced a "strict travel ban between Scotland and the rest of the UK".
Northern Ireland was already set to enter a six-week lockdown from Boxing Day.
The UK government announced a further 27,052 confirmed COVID-19 cases across the UK on Saturday, with 534 more people having died within 28 days of testing positive for the virus.
Under the new Tier 4 measures in southeast England, all non-essential shops, gyms, and hairdressers are closed, with people ordered to stay home apart from limited exceptions such as work (if people cannot work from home), education, childcare and exercise.
However, communal worship is allowed to continue under the toughened restrictions.
Mr Johnson ditched his previous plans for a five-day relaxation of COVID rules over Christmas and told people in Tier 4 areas they will no longer be allowed to form "Christmas bubbles" with other households.
Meanwhile, outside the South East, people will still be allowed to mix in private homes with up to two other households, but now only on Christmas Day.
"When the virus changes its method of attack, we must change our method of defence," Mr Johnson said.
"As your prime minister, I sincerely believe there is no alternative open to me."
The Tier 4 measures in the South East will be in place for an initial two-week period, with a review on 30 December, and Mr Johnson stressed that people must stick to the rules for the New Year period.
Those in Tier 4 areas are being ordered not to stay overnight away from home or travel abroad for holidays, while people in other areas are being advised not to enter Tier 4 areas - and not to travel generally if they can avoid it.
Pictures on social media on Saturday evening showed large crowds at London's St Pancras station as people tried to leave the capital before Tier 4 came into effect.
Seats on services to Newcastle appeared to be heavily booked.
A woman who did not wish to be named said she and her partner had made a "split decision" to take their young son to her parents' home on the coast.
"We just made the decision to leave based on the fact that my parents said come, and we couldn't bear the thought of no fresh air and a toddler going rogue round a small flat for the foreseeable," she said.
Under England's new highest level of restrictions, only one person from a household can meet with another person from a different household outside, although exemptions will continue for support bubbles and childcare bubbles.
Those who are deemed to be clinically extremely vulnerable in Tier 4 areas will be told to follow the same guidance as during November's England-wide lockdown, which urged those people to stay at home as much as possible apart from outdoor exercise or to attend health appointments.
Downing Street suggested schools would still reopen in Tier 4 areas after the Christmas holidays.
The areas now in Tier 4 are:
East of England (Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire, Essex excluding Colchester, Uttlesford and Tendring)
London (all 32 boroughs and the City of London)
Rother and Hastings
Surrey (excluding Waverley)
The action was prompted by the government's fears over a new strain of coronavirus - known as VUI 2020/01 - which is believed to spread more quickly than the original strain and is judged to be the reason for rapidly rising infection rates in the South East.
Mr Johnson said analysis by the government's New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) suggests the new strain could increase the R rate by "0.4 or greater" and might be "up to 70% more transmissible".
"While we are fairly certain the variant is transmitted more quickly, there is no evidence to suggest that it is more lethal or causes more severe illness," the prime minister said.
"Equally there is no evidence to suggest the vaccine will be any less effective against the new variant."
COVID-19 cases have been nearly doubling in London over the past few weeks, while in Kent cases have continued to rise despite the county having been under Tier 3 measures since the end of last month's lockdown.
This has led to fears that Tier 3 measures are unable to contain the new strain and more needs to be done in London and the South East.
Sir Patrick Vallance, the government's chief scientific adviser, said that by December over 60% of infections in the capital had been the new variant, which is thought to have emerged in mid-September.
England's chief medical officer, Professor Chris Whitty, urged people not to "unwittingly" spread the new strain to other parts of the country.
Asked if people considering leaving Tier 4 areas on Saturday evening should unpack their bag and stay at home, he replied: "My short answer would be 'please unpack it at this stage'."
The new strain has been detected in two other countries other than the UK, albeit in a very small number of cases.
The prime minister urged people to continue to get COVID vaccinations as he revealed 350,000 people across the UK had already had their first dose of the Pfizer/BioNTech jab.
"Yes, Christmas this year will be very different, but we must be realistic," Mr Johnson said.
"We are sacrificing our chance to see loved ones this Christmas, so we have a better chance of protecting their lives so we can see them at future Christmases."
He suggested people "lift a glass to those that aren't there" over the festive period.
The government currently has no plans to recall parliament to allow MPs to vote on the new measures, although some senior Conservatives demanded the House of Commons return from its Christmas recess to have a say on the restrictions.
Labour leader Sir Keir Starmer said he supported the fresh government action in the face of rising infection rates, but added: "Millions of families across the country are going to be heartbroken by this news - having their Christmas plans ripped up.
"And I'm really frustrated because I raised this with the prime minister on Wednesday and he dismissed that and went on to tell people to have a merry little Christmas - only three days later to rip up their plans.
"I think the British public is entitled to more decisive leadership than that. They're getting confusion where they need certainty."
London mayor Sadiq Khan described the announcement of a strict new lockdown for the capital as a "bitter blow".
"Our city faces its toughest Christmas since the war - and we will all need to pull together to see us through this terrible period," he said.
"Please follow the new rules - lives depend on it."