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 and Scotland.
In 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.
In areas of England 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 Wales (two households) and Scotland (three households up to a maximum of eight people) Christmas bubbles will also only be allowed on 25 December. In both countries, people are being advised not to form a Christmas bubble, even though they are permitted.
In Scotland, travel to and from other parts of the UK will not be allowed over the festive period.
Northern Ireland has a window of 23 to 27 December for its bubble, with days at either end for people who are travelling into or out of the nation.
Bubbles can meet each other:
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.
In England if you have formed a support bubble with another household, that counts as one household. In Scotland, guidance says any Christmas bubble should contain no more than one “extended household”.
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.
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, in areas where mixing is still possible.