Boris Johnson has warned "a smaller Christmas is going to be a safer Christmas" as he urged the public to see festive bubbles of up to three households as "maximums" and not "targets to aim for".
The prime minister told a Downing Street news conference that, following discussions over the last two days, the four nations of the UK had "collectively" agreed to keep the relaxation of COVID restrictions over Christmas.
But, urging restraint, Mr Johnson said all four nations were now offering the same message that: "A smaller Christmas is going to be a safer Christmas, and a shorter Christmas is a safer Christmas."
However, the prime minister's suggestion of unity among the UK's leaders on keeping the original Christmas rules the same in law was soon undone.
The Welsh government later announced that it would be putting its new guidance on limiting Christmas gatherings to just two households into legislation.
"Ministers decided this afternoon that we will be amending the regulations in Wales," a spokesman said.
"For 23 to 27 December the restriction on two households meeting will be part of Welsh law.
"It makes it easier, so we don't have the position where the law says one thing and the guidance says something else."
Scotland's First Minister Nicola Sturgeon had also earlier recommended that those people forming a Christmas bubble should only meet up on one day and not stay overnight "unless it is unavoidable".
In Northern Ireland, First Minister Arlene Foster said the public must take "all and every precaution" at Christmas and proposals for further restrictions would be brought forward on Thursday.
For five days between 23 and 27 December, people across the UK had initially been told they could mix in private homes with those in their "Christmas bubble" of up to three households, while travel restrictions would also be lifted.
These rules remain the same for people in England.
At the Number 10 news conference, Mr Johnson admitted that the "overall situation" is now "worse and more challenging" than when the loosening of restrictions over the festive period was first announced last month.
Mr Johnson said it would not be right to "criminalise" people who have already made plans and "simply want to spend time with their loved ones".
But he said he was asking people to "think hard and in detail about the days ahead and whether you can do more to protect yourself and others", as he urged the public to act well within the rules.
Mr Johnson also advised the public to:
reduce the number of people you are in contact with to the lowest possible during the five days before the Christmas loosening of restrictions begins on 23 December
avoid travel from a high prevalence to a low prevalence area, if possible
avoid overnight stays away from home if you can
think about waiting to see elderly relatives until after they have been vaccinated
avoid crowds in the Boxing Day sales
don't gather in large groups to celebrate the New Year
The prime minister said there had been "worrying rises" in COVID infections in some parts of the country, with London and parts of Essex and Hertfordshire having been put into Tier 3 measures earlier this week.
"So have yourselves a merry little Christmas and, I'm afraid, this year I do mean little," Mr Johnson said.
"But with the vaccine, and all the other measures that we are taking, we do know things will be better in this country by Easter.
"And I'm sure that next year Christmas will be as normal for every family in the country."
The prime minister stressed that he did not want to "cancel Christmas" or "ban it" and that the five-day and three-household rules remain "a highly prescriptive approach".
"I don't think there's been anything like it... probably not since Cromwell's time - heaven knows when we have had something like this before," he said.
England's chief medical officer, Professor Chris Whitty, told the public to "keep it small, keep it short, keep it local, and think of the most vulnerable people" when gathering over Christmas.
Referring to the rollout of COVID vaccines, Prof Whitty added: "We are tantalisingly close to the stage where anybody who gets into trouble as the result of actions this Christmas would have been protected in the very near future.
"So, it is very important that people think about that when they make decisions over the next few weeks."