Coronavirus restrictions on social mixing and travel will be eased across the UK over Christmas to allow up to three households to meet indoors.
The "Christmas bubble" will be in force from December 23 to 27, the leaders of all four UK nations agreed on Tuesday night.
Scientists said the plan would "throw fuel on the Covid fire" and lead to more hospital admissions and deaths.
But the Welsh executive said it agreed to a five-day amnesty to prevent a "free-for-all" where people simply ignored the guidance.
The deal was reached at a meeting of the UK government's emergency committee, Cobra.
"As 2020 draws to a close, we recognise it has been an incredibly difficult year for us all," a joint statement said.
"We have all had to make significant sacrifices in our everyday lives, and many religious and community groups have already had to change or forgo their customary celebrations to slow the spread of coronavirus and save lives," it said.
"This cannot be a 'normal' Christmas. But as we approach the festive period, we have been working closely together to find a way for family and friends to see each other, even if it is for a short time, and recognising that it must be both limited and cautious."
England will re-enter its tier system after lockdown ends on December 2.
However, under the new Christmas plans, people will be able to travel between tiers and across the UK, as long as they meet only with those in their exclusive bubble of three households. Those going to and from Northern Ireland will be allowed an extra day to travel either side of the five-day period.
The guidance says a bubble of three households will be able to stay overnight at each other's home but would not be able to visit shops, restaurants and pubs.
And once a bubble is formed, it must not be changed or be extended further.
Hugging will be allowed but ministers said people should take care, particularly around the elderly and vulnerable.
Prime Minister Boris Johnson urged people not to “throw caution to the wind. The virus doesn’t know it’s Christmas, and we must all be careful”.
The warning comes as authorities fear people will let down their guard following news that there are three effective vaccines on the way.
All four UK nations have agreed that from the 23rd to the 27th of December, you will be able to form a Christmas bubble of no more than three households.
— Boris Johnson (@BorisJohnson) November 24, 2020
https://t.co/5qLRcLBP8W pic.twitter.com/boonZD27Q8
But doctors from the British Medical Association (BMA) immediately raised concerns.
BMA council chairman Chaand Nagpaul said the move “will almost certainly carry the risk of a rise in infection rate and possibly more hospitalisation and deaths”.
Prof Andrew Hayward, an infectious disease expert and a member of the government's scientific advisory committee, said the move would "throw fuel on the Covid fire".
He told BBC's Newsnight: "It will definitely lead to increased transmission and it's likely to lead to a third wave of infection, with hospitals being overrun and more unnecessary deaths.
"Covid is a disease that thrives on social contact, especially close duration, close proximity contact in households."
Wales's First Minister Mark Drakeford said people would have ignored a blanket ban on meeting up at Christmas.
He told BBC Radio 4's Today programme: "Many people simply would have taken it into their own hands. It is better to have a modest relaxation with clear rules around it than having a free-for-all."
The UK has one of the highest tolls from the coronavirus pandemic, reaching 55,838 deaths within 28 days of a positive test, according to the latest data.
"The virus won’t take time off, so please be cautious," Scotland's First Minister Nicola Sturgeon said. "If you can, stay at home with your own household."
British politician quizzed on Christmas celebrations when Eid and Diwali were 'cancelled'
Also on Tuesday, Health Secretary Matt Hancock said Christmas was singled out for a reprieve from coronavirus rules because it is the UK’s most important holiday.
Asked about the importance of Christmas over other religious holidays including Eid and Diwali, Mr Hancock told MPs that Christmas is the “biggest national holiday we have”.
Muslims, Sikhs and Hindus have not been able to enjoy their religious celebrations with their families this year.
This year, Eid Al Fitr took place during the UK's spring lockdown. Eid Al Adha was also affected by coronavirus restrictions with many celebrations cancelled in parts of the country.
Many in the British Muslim community were dismayed because food had already been prepared and gifts were ready.
Mr Hancock said: “I'm very sensitive to this point.
“And we did think about it and we were engaged and we have discussed it and the conclusion that we've come to, which I agree with very strongly, is that Christmas is a national holiday and it's the biggest national holiday we have.
“Of course, it has a particular importance for Christians but it is a national holiday for everybody in this country."
Chancellor Rishi Sunak, a practising Hindu, previously defended the government's plans to lift restrictions temporarily over Christmas.
He said: "Of course this is a secular country. But Christmas is also a national time when, regardless of whether you're going to midnight Mass or to church, it's a time when most people have time off work, we have holidays, state holidays, it is a time when everyone hangs out.
"I'm Hindu, but I'm also equally going to be tucking in to my Christmas meal – the kids are excited about that."
