A stringent new lockdown has come into force in London and large parts of the east and south-east of England.
Prime Minister Boris Johnson said tough rules are needed after a new variant of coronavirus caused cases to soar.
Nearly 18 million people entered tier four restrictions at midnight meaning they will not be able to mix with other households over Christmas.
In the rest of England, Scotland and Wales relaxed indoor mixing rules are cut from five days to Christmas Day.
The toughest restrictions – similar to England’s second national lockdown – applies to all areas in the South East currently in tier three, covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It also applies in London (all 32 boroughs and the City of London) and the East of England (Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire and Essex (excluding Colchester, Uttlesford and Tendring).
Tier four restrictions:
People elsewhere will be advised not to travel into a tier four area.
The changes for England, announced at a Downing Street briefing on Saturday, will last for two weeks with the first review due on 30 December.
In Scotland, Covid restrictions will only be relaxed on Christmas Day, with mainland Scotland being placed under the tightest restrictions from Boxing Day.
A ban on travel to the rest of the UK will also apply over the festive period.
In Wales, First Minister Mark Drakeford said the country will be placed into lockdown from midnight.
In Northern Ireland, no changes have been made to Christmas restrictions, with three households allowed to meet from 23 to 27 December. The country is set to enter a six-week lockdown from 26 December.
The PM’s announcement came just days after he defended plans to relax restrictions for five days during the festive period – despite calls by some in the medical profession to scrap the change.
Mr Johnson told the Downing Street briefing on Saturday that he knew how “disappointing” the news would be, but said he believed there was no alternative.
Scientists have warned that a new variant of the coronavirus variant is more infectious and spreading more rapidly leading Mr Johnson to say the government had to “change our method of defence”.
The prime minister said analysis suggested the new variant could increase the R number – which indicates if an epidemic is growing or shrinking – by 0.4 or more.
Labour leader Sir Keir Starmer, who raised the prospect of tougher restrictions in the Commons on Wednesday, said he was “really frustrated”.
“Millions of families will be heartbroken by having Christmas plans ripped up,” Sir Keir said.
Following Saturday’s announcement, the Dutch government announced it was banning passenger flights between the Netherlands and the UK from 05:00 GMT Sunday, until 1 January at the latest.
The steep increase in the proportion of coronavirus cases linked to this new variant is strong evidence that it is driving transmission.
In London, 28% of cases were as a result of this new mutation in mid-November, but that has now increased to more than 60%.
It may explain why, during the second lockdown, cases started to increase in London, while in Kent the tier three measures appear to have had little impact in recent weeks.
As England’s chief medical adviser Prof Chris Whitty says, this is a bad moment, but there is also some hope.
Mutations happen all the time – there have been thousands of variations to this coronavirus since it emerged – and there is nothing to suggest this causes more serious illness or will hamper the effectiveness of the vaccine.
The prime minister announced 350,000 people had been given the first dose of the vaccine in the first two weeks of the programme.
In the coming weeks, the number of GP-led vaccination clinics should increase six-fold, while approval of a second vaccine made by Oxford University could pave the way for mass vaccination centres to be set up in sports stadiums and conference centres.
That could see two million people a week being vaccinated. Within a matter of months all the over-65s could have been offered a jab. This could then start to feel very different.
But for now, the slog of the pandemic continues – and for many it just got harder.
Mr Johnson is also facing criticism from within Conservative Party ranks.
Mark Harper, chairman of the Covid Recovery Group of Tory MPs – which has been highly critical of the government’s strategy – called for Parliament to be recalled so MPs could debate and vote on the changes.
British Chambers of Commerce director general Adam Marshall asked what support would be available for companies whose cash flow projections “have once again been thrown into chaos”.
In other developments: