The government will ease restrictions and legally allow up to three households to mix between December 23 and 27.
But while the law will remain the same across the UK, the advice given to people in the different nations will be different.
In Scotland, first minister Nicola Sturgeon said her “strong recommendation” was that people “stay within your own household and your own home”.
In Wales, first minister Mark Drakeford said “only two” households should mix over Christmas rather than three.
“None of us wants to be ill this Christmas. And we don’t want to give coronavirus to our close family or friends,” he said.
The prime minister has been under pressure to scrap the legal relaxation amid fears it will lead to a surge in infections.
But speaking during PMQs on Wednesday, Johnson said it would “proceed”.
“We don’t want to criminalise people’s long-made plans,” he told MPs.
Johnson said a “unanimous agreement” had been reached between devolved leaders on keeping the law the same.
But he said while the law would still permit people to mix three households, the public should “exercise extreme caution” over Christmas.
“We can celebrate it sensibly but we have to be extremely cautious in the way we behave,” he said.
“It’s absolutely vital that people should – at this very, very tricky time – exercise a high degree of personal responsibility, especially when they come into contact with elderly people, and avoid contact with elderly people wherever possible.”
Keir Starmer, the Labour leader, said Johnson’s “slowness” to respond to the pandemic had repeatedly “led to more deaths, a longer lockdown and a deeper economic damage”.
He said Johnson’s decision to push ahead with a relaxation of the restrictions over Christmas was the prime minister’s “next big mistake”.
In a rare joint editorial, the British Medical Journal and Health Service Journal warned on Tuesday relaxing the rules over Christmas would “cost many lives”.
In Canada, the number of Covid cases surged in the weeks after restrictions were relaxed for Thanksgiving.
Chris Hopson, chief executive of NHS Providers, which represents hospital trusts in England, also pointed to a spike in cases in the United States after Thanksgiving.
“The number of cases has gone up by 25 per cent, the number of hospitalisations has gone up by 20 per cent in a fortnight, and the number of deaths has gone up by 60 per cent in a fortnight,” he told the BBC.