Up to three households will be able to meet up during a five-day Christmas period of 23 to 27 December, leaders of the four UK nations have agreed.
People can mix in homes, places of worship and outdoor spaces, and travel restrictions will also be eased. But a formed "Christmas bubble" must be "exclusive" and would not be able to visit pubs or restaurants together.
The leaders urged people to "think carefully about what they do" to keep the risk of increased transmission low. They added 2020 "cannot be a normal Christmas" but family and friends will be able to see each other in a "limited and cautious" way.
The measures will see travel restrictions across the four nations, and between tiers and levels, lifted to allow people to visit families in other parts of the UK. Anyone travelling to or from Northern Ireland may travel on the 22 and 28 December, but otherwise travel to and from bubbles should be done between the 23 and 27. People will not be able to get together with others from more than two other households, and once a bubble is formed, it must not be changed or be extended further.
The guidance says a bubble of three households would be able to stay overnight at each other's home but would not be able to visit hospitality, theatres or retail settings.
The leaders of England, Scotland, Wales and Northern Ireland reached the agreement at a meeting on Tuesday afternoon.
In a joint statement, they said: "Even where it is within the rules, meeting with friends and family over Christmas will be a personal judgement for individuals to take, mindful of the risks to themselves and others, particularly those who are vulnerable.
"Before deciding to come together over the festive period we urge the consideration of alternative approaches such as the use of technology or meeting outside."
Published guidance for England gives further details of the rules:
- People can continue to meet people outside their Christmas bubble outdoors according to the rules in the tier where they live
- Children under the age of 18 whose parents do not live together may be part of both parents' Christmas bubbles
- Existing support bubbles count as one household towards the three household limit
- People are allowed to form a different Christmas bubble from the people they live with normally - they can choose to stay with different people for this period
- If a care home resident is able to leave their home, they can form a bubble with one other household - but should not form a three-household bubble. However, visits out of care homes should only be considered for residents of working age because of the risks
- Students are considered to be part of the household to which they have returned
Scientists say a typical Christmas gathering at home is the type of environment where infections can spread.
The guidance also advises people to take precautions when meeting their Christmas bubble such as washing hands frequently and opening windows to clear potential virus particles.
In a video message from Downing Street, the prime minister described the agreement as a "special, time-limited dispensation", saying: "This year means Christmas will be different."
Boris Johnson said people must make a "personal judgment" about the risk of who they form a bubble with or if they visit elderly relatives., adding: "Many of us are longing to spend time with family and friends... And yet we can't afford to throw caution to the wind."
Wales' First Minister Mark Drakeford said it was "not an instruction to travel, it's not an instruction to meet with other people. People should still use a sense of responsibility".
Scotland's First Minister Nicola Sturgeon added: "The virus is not going to be taking Christmas off, so although we want to give a little bit of flexibility for Christmas we are still urging people to be very cautious and to use this flexibility responsibly and only if you think it is necessary."
Northern Ireland's First Minister Arlene Foster said she hoped people would have space to plan, adding: "We of course recognise how important Christmas time is for so many people."
Deputy First Minister Michelle O'Neill urged people to "be responsible", saying while they wanted to mark Christmas after such a "desperate" year the relaxations would increase opportunities for the virus to spread.
She added it was hoped that an alignment with rules in the Irish Republic could be achieved.