Prime Minister Boris Johnson said the responsibility for Christmas will be on the people rather than the law.
It follows a decision by the devolved nations earlier on Wednesday not to change the plan to relax restrictions from December 23-27 to allow up to three households to mix over the festive period.
Mr Johnson told a press briefing that a colossal effort had taken place to bring the virus under control, but to “be frank with you, we’re already seeing worrying rises in some parts of the country.”
He told the country to “exercise extreme caution” and “the greatest personal responsibility”. He said he did not want to “cancel Christmas”.
“While it would not be right to criminalise people who have made plans and simply want to spend time with their loved ones, we’re […] asking you to think hard, and in detail about the days ahead, and whether you can do more to protect yourself and others.
“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.”
He said the three household / five days relaxations were “maximums, not targets to aim for”.
“Have yourself a merry little Christmas and I’m afraid this year, I do mean little,” he said.
England’s Chief Medical Officer Chris Whitty repeated the message to keep it small, short and local as well as “think about the most vulnerable.”
He said the vaccine was still two to three months away from being rolled out to the general population.
“We are tantalising close to the stage where anybody who gets into trouble as a result of actions this Christmas would have been protected in the very near future.
“So it is very important people think about that when they make decisions.”