The government has scrapped plans to relax Covid rules at Christmas for London and much of south-east England, where tier four restrictions will now apply.
In other regions of England - in tiers one to three - Christmas bubbles will now only be allowed on 25 December.
Similar plans have been announced in Wales, Scotland and Northern Ireland.
How many households can meet at Christmas?
In England's tier four areas, residents will only be allowed to celebrate Christmas with members of their own household and support bubbles. They will not be allowed to travel to other tiers to see family and friends.
More parts of south-east and eastern England will be placed in tier four from 00:01 GMT on Saturday 26 December. Residents will be asked to stay at home and all non-essential shops must close.
In areas in tiers one, two and three Christmas bubbles will no longer be across five days. Instead, three households coming together will now only be allowed on Christmas Day.
People in these tiers are also being asked to stay local. They will not be allowed to host people who live in a tier four area.
In Scotland, travel to and from other parts of the UK will not be allowed over the festive period.
Northern Ireland has also cut its Christmas bubble window to a choice of one day between 23 and 27 December. It is being flexible to make allowances for people who are working on Christmas Day.
What are Christmas bubbles allowed to do?
Bubbles can meet each other:
- In each other's homes
- At a place of worship
- In an outdoor public space or garden
In England, the government advises people to avoid travelling to other areas and only to stay overnight with someone else if absolutely necessary.
You cannot go to the pub with members of your Christmas bubble.
People who are self-isolating should not join a Christmas bubble. If someone tests positive, or develops coronavirus symptoms up to 48 hours after the Christmas bubble last met, everyone has to self-isolate.
Can all my children come home for Christmas?
Under-18s whose parents live apart can join two Christmas bubbles, so they can see both parents without being counted as part of another household.
University students who travel home will be counted as part of their family household straight away.
But if a family has three or more grown-up children not at university, they cannot all form a Christmas bubble with their parents, even in areas where mixing is still possible.
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Correction: A previous version of this piece said government rules allowed people to stay overnight with someone else if absolutely necessary. The rules actually said "do not stay overnight", but that "if necessary, you can stay in private rented accommodation with members of your household, or your Christmas bubble".