Boris Johnson has cancelled Christmas for millions of people across London and south-east England, who will go up to a new Tier 4 of restrictions.
Further measures to halt the spread of coronavirus have also been introduced across Wales and Scotland.
Here the PA outlines what the new Christmas rules are, and what new restrictions are coming into force.
Not completely, but the PM has effectively cancelled it for nearly 18 million in London, the South East and East of England.
These areas will move to a new Tier 4, effectively returning to the lockdown rules of November, and will not be able to join or form Christmas bubbles.
For everyone else in England, the Christmas “bubble” policy – allowing up to three household to meet up over the holiday period – will be severely curtailed to apply on Christmas Day only.
So people outside the new fourth tier will be able to celebrate Christmas, but for just one day.
– What is Tier 4?
Under the new Tier 4, which comes into effect on Sunday, non-essential shops, gyms, cinemas, hairdressers and bowling alleys will be forced to close for two weeks.
Pubs, bars and restaurants will be closed except for takeaways, drive-throughs and deliveries.
People should not mix with anyone outside their own household, although support bubbles will remain in place and two people from different households can meet in public outdoor spaces.
Mr Johnson also said that everyone in Tier 4 areas should work form home if they can, not enter or leave Tier 4 areas and not stay away from home overnight.
People will be allowed to travel for education, childcare and to go to work if they cannot work from home – and they will be permitted unlimited outdoor exercise.
– Where is going into Tier 4?
The new Tier 4 restrictions will apply in all Tier 3 areas in the South East – covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It will also apply 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).
The move comes after scientists on the Government’s New and Emerging Respiratory Virus Threats Advisory Group (NervTag) concluded that the mutant strain identified by Public Health England – known as VUI2020/01 – was spreading more quickly.
Mr Johnson told a Downing Street briefing that early analysis showed the new strain could increase the reproductive rate by 0.4 or more and that it may be up to 70% more transmissible than the old variant.
The Prime Minister was advised of the group’s conclusions at a meeting with ministers on Friday evening, and the new regulations were signed off by the Cabinet in a conference call on Saturday lunchtime.
– What should people going into Tier 4 do if they have plans for Christmas?
Sadly, England’s chief medical officer Professor Chris Whitty said people considering leaving Tier 4 areas now should unpack their bag and stay at home.
He advised people should minimise contact with grandparents, keeping it at a distance and for the “shortest period you can”.
– What will happen if people break the rules?
Mr Johnson did not confirm whether police would be told to stop people travelling home over the festive period or knock on doors on Christmas Day.
He said that officers had so far done an “amazing job” of “light touch” policing, and that the public had been generally compliant.
– Are other parts of the UK changing their rules?
Yes, Scotland and Wales have both strengthened their restrictions.
Scotland’s First Minister Nicola Sturgeon announced a “strict travel ban” over the festive period.
The travel ban between Scotland and the rest of the UK will remain in place throughout the Christmas holidays and bubbles will only be allowed on Christmas Day.
All of the Scottish mainland will be placed into Level 4 restrictions from Boxing Day.
The restrictions will be reviewed after two weeks, Ms Sturgeon added.
– What about Wales?
The Welsh Government has announced that lockdown restrictions will come into effect in Wales from midnight on Saturday, while festive bubbles will only apply on Christmas Day.
First Minister Mark Drakeford said the rules would mean non-essential retail, close contact services, gyms and leisure centres and hospitality would close at “end of trading” on Saturday.
Stay-at-home rules will also come into effect from midnight.
– Is Northern Ireland changing its rules as well?
PA understands that there is no immediate plan to change the Christmas Covid-19 restrictions in Northern Ireland, despite rules being tightened in other parts of the UK.
Sources said meetings between the Executive and the UK Government had been ongoing all day.
First Minister Arlene Foster, deputy First Minister Michelle O’Neill, Health Minister Robin Swann and chief medical officer Michael McBride held discussions on the issue on Saturday.