The relaxed restrictions introduced across the UK for five days over Christmas do not mandate social distancing between members of up to three households allowed to gather for the festive period.
Prof Whitty said it was best not to hug vulnerable elderly relatives “if you want them to survive to be hugged again”.
The CMO said that he would personally be spending Christmas Day “on the wards”, but recognised that millions of others would want to be with their loved ones.
“Would I want someone to see their family? Of course, that’s what Christmas Christmas is about,” said Prof Whitty.
“But would I encourage someone to hug and kiss their elderly relatives? No I would not.
“It's not against the law…. you can do it within the rules that are there.
“But it does not make sense because you could be carrying a virus and if you’ve got an elderly relative, that would not be the thing you'd want to do in a period where we're running up to that point where actually we might be able to protect older people.
“So I think people just have to have sense. The fact that you can do something - and this is true across so many other areas of life - doesn't mean you should.”
Prof Vallance said that anyone forming a Christmas bubble should “keep numbers down, don't do things that are unnecessary, try to make sure that you avoid behaviours that will spread the disease, make sure that if you're in a house with other households that you've got it well ventilated and you take the precautions, keep distance where you can”.
And he agreed: “I think hugging elderly relatives is not something to go out and do, it will increase the spread to a vulnerable population.”
Official rules for Christmas, set out earlier this week, say that members of festive bubbles should “keep socially distanced from anybody you do not live with as much as possible”. But they do not rule out hugging or being close to relatives.
Mr Johnson told the press conference that families should be “common-sensical” about how they take advantage of the additional freedoms granted during the festive period.
“Until the vaccine comes on stream, we are not out of the woods yet,” said the PM. “And we have to be very, very vigilant. Everybody's individual behaviour at Christmas will matter a great deal.”