Leaders of the four UK nations have warned people to be cautious of the risk of spreading coronavirus when rules are relaxed over Christmas.
Up to three households will be allowed to form a “Christmas bubble” from 23 to 27 December.
Prime Minister Boris Johnson told people to use their “personal judgment” on whether to visit elderly relatives.
A scientific adviser to the government said the relaxation of rules amounted to “throwing fuel on the Covid fire”.
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.
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.”
Mr Johnson said people must consider the risks of who to form a bubble with and whether or not to visit elderly or vulnerable 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.
“The virus doesn’t know it’s Christmas and we must all be careful.”
The leaders of England, Scotland, Wales and Northern Ireland reached the agreement at a meeting on Tuesday afternoon.
In a joint statement, they urged people to “think carefully about what they do” to keep the risk of increased transmission low.
They said: “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.”
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.
Published guidance for England gives further details of the rules:
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.
Some scientists have warned that the relaxation of Covid restrictions over the festive period could spark another wave of infections and further deaths.
Prof Andrew Hayward, director of the UCL Institute of Epidemiology and Health Care, and a member of the government’s Sage committee, told BBC Newsnight that allowing families to meet up over Christmas amounted to “throwing fuel on the Covid fire”.
He said it would “definitely lead to increase[d] transmission and likely lead to third wave of infections with hospitals being overrun, and more unnecessary deaths.”
Prof Hayward said while you cannot ban Christmas, he called for clearer messaging to families about the “dangers” of socialising and inter-generational mixing.
Paul Hunter, professor of medicine at the University of East Anglia, suggested the relaxation of restrictions at Christmas will “almost inevitably” lead to an increase in transmission.
But he said: “Providing that the new tier system is better managed than in October, any increase in cases could be relatively short-lived.
“After Christmas we will still have to live through a few more months of restrictions at least.”
Jillian Evans, the director of health intelligence at NHS Grampian, said the easing of restrictions over Christmas would cost lives.
“We’ve got winter weather, we know that people are more susceptible to infection over the colder period, and we’ve got a festive period where people will be socialising,” she said.
“Those are facts, and I would rather be honest and tell you that those are the facts, and be truthful about it so people can understand the risks that they’re taking.”
What to do about Christmas divides opinion.
Increased mixing indoors will certainly mean there is greater transmission of the virus.
But, as chief medical adviser Prof Chris Whitty said on Monday, there is a balance to be struck between the harm the virus can cause and the societal and economic impacts of trying to control it.
He called for a “public-spirited approach”.
By that he means adhering to the restrictions in the lead-up to Christmas, being responsible with the opportunity the relaxation gives people, and then immediately switching back to compliance.
If that happens, any impact could be minimised – and, of course, it will be up to individuals to decide just how much they mix within the rules.
These are very fine judgement calls by ministers.
They hope Christmas will provide respite and help steel the public for what is clearly going to be a long, hard winter.
They also feel they have little choice, believing large numbers of people would ignore pleas not to mix – and this way they can provide advice on how to enjoy Christmas as safely as possible.
But there is also the risk by sanctioning it there will be more mixing than there would have otherwise been.
Transport Secretary Grant Shapps has said Christmas travellers should plan journeys carefully and prepare for restrictions on passenger numbers to allow for social distancing.
On Tuesday, the government recorded another 608 UK deaths within 28 days of a positive Covid test, as well as a further 11,299 cases of people testing positive for the virus.
Gyms and non-essential shops in all parts of England will be allowed to reopen from 2 December under a strengthened three-tiered system.
Areas will not find out which tier they are in until Thursday – and the decision will be based on a number of factors including case numbers, the reproduction rate – or R number – and pressure on local NHS services.