Boris Johnson has urged people to "think hard" about how they spend Christmas.
The prime minister issued the plea during a Downing Street press conference on Wednesday (December 16).
But he warned the overall situation was "worse" and "more challenging" than hoped when the relaxation was decided.
As a result, people are being advised to have small celebrations on December 25.
Mr Johnson said the message was: "A smaller Christmas is going to be a safer Christmas and a shorter Christmas is a safer Christmas."
He added that three households mixing over the permitted period were "maximums and not targets to aim for".
Mr Johnson also advised people to minimise contacts before seeing loved ones at Christmas, and to avoid travelling from high prevalence areas to ones that are not so badly affected.
He said: "If you are visiting others over Christmas we're asking you in the five days beforehand, as early as this Friday, to reduce the number of people you are in contact with to the lowest possible.
"If possible don't travel from a high prevalence to a lower prevalence area and avoid staying away from home overnight if you can."
Mr Johnson added: "So, have yourselves a merry little Christmas - and, I'm afraid this year I do mean little."
But he shared hope after the vaccine roll-out which has seen more than 130,000 people in the UK receive the jab in the first week.
Mr Johnson said: "But with the vaccine, and all the other measures we are taking, we do know that things will be better in this country by Easter.
"And, I'm sure that next year Christmas will be as normal as usual for every family in the country."
Mr Johnson has said that it would be "frankly inhuman" to "ban Christmas".
He said: "I want to be clear, we don't want to ban Christmas, to cancel it, I think that would be frankly inhuman and against the instincts of many people in this country.
"But what we are saying is that that guidance, the three households and the five days, that should really be regarded as a maximum, those are the outer limits."
Chief medical officer for England professor Chris Whitty said it was key to keep it small, keep it short and keep it local.
He made a comparison with driving at 70mph on an icy road and acknowledged if every household takes it to the limit during Christmas there would be a "significant problem".
He said: "This is the equivalent of us saying these are icy and treacherous conditions, if you wish to have a driving analogy, we should be doing the minimum necessary still within the law.
"Just because you can doesn't mean you should."
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