Boris Johnson has told families to keep Christmas celebrations "little" as he announced tougher guidance for the festive period.
In a significant shift, people are now advised to keep Christmas short, small and local, with particular concern for vulnerable people.
Speaking at a No10 press conference, Mr Johnson urged people to "exercise extreme caution" over the festive period and consider isolating from Friday if they can.
People are advised to keep it local, not travel from a high to low risk area and not stay overnight if possible under the new guidance.
The Prime Minister said the coronavirus situation was "worse, more challenging" that when the rules were drawn up - but the Government did not want to "criminalise" people for seeing their loved ones over Christmas.
Mr Johnson told the public: "We’re keeping the laws the same but we all want to send the same message - a smaller Christmas is going to be a safer Christmas, and a shorter Christmas is a safer Christmas.”
The current rules say up to three households can form an exclusive Christmas bubble between December 23-27.
People are allowed to travel across the country - regardless of their tier - to spend the five-day period with family.
The guidance is now being toughened following pressure from medics fuelled by soaring coronavirus rates in parts of the country.
Setting out the new plans for England, Mr Johnson explained: "When we say three households can meet on five days I want to stress these are maximums and not targets to aim for.
"It's always going to be safest to minimise the number of people you meet.
"That means 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 also urged people to avoid the Boxing Day sales and other large crowds.
"And no one should be gathering in large groups to see in the New Year," he said.
"If you have an elderly relative you might want to delay seeing them until they have been vaccinated."
The PM said it would be "inhuman" to cancel Christmas and ministers were trying to come up with "a maximum that we think is sensible".
He said: "Have yourselves a merry little Christmas - and I'm afraid this year I do mean little.
"But with the vaccine, and all the other measures that we are taking, we do know that things will be better in this country by Easter."
Prof Whitty said people should treat the rules as a limit not a target.
He warned: "If everybody pushes it to the limit that would be a significant problem."
The latest advice comes after Mr Johnson told MPs that the Christmas rules had been agreed "in principle" across the UK - but it emerged soon after Wales and Scotland plan to diverge on some guidance.
In Wales, the number of households that can bubble together has been reduced to two, rather than three.
The country will also be plunged into a full lockdown on December 28, First Minister Mark Drakeford said.
Nicola Sturgeon advised Scots not to meet indoors at all if possible but if it was "essential" to meet inside then she advised people to meet for only one day and to not stay overnight.
A joint statement from the Government and the leaders of Scotland, Wales and Northern Ireland is due to be published this afternoon.
