Boris Johnson has warned families they must make a “personal judgment” about the risks of coronavirus to vulnerable loved ones when forming a Christmas bubble.

The Prime Minister urged the public to “think carefully” over the festive period after it was confirmed that three households will be able to mix from December 23 to 27.

The UK Government and devolved administrations have agreed a joint plan to relax social distancing rules over the festive period, allowing friends and family to hug for the first time in months.

But a member of the Government’s Scientific Advisory Group for Emergencies (Sage) warned that the planned easing of measures could lead to a third wave of the pandemic.

Mr Johnson acknowledged the measures would not add up to a “normal Christmas” and urged people to exercise caution, particularly when meeting with the elderly or the vulnerable.

“We can’t afford to throw caution to the wind. The virus doesn’t know it’s Christmas and we must all be careful,” he said in a video message posted on Twitter.

“I know this doesn’t equate to a normal Christmas and it won’t work for everyone. And it is up to each of us to think carefully about how we use this time-limited special dispensation.

“The virus has not gone away and families will need to make a personal judgment about the risk of forming a bubble with or visiting elderly relatives and the vulnerable.”

A joint statement issued by the four UK governments said they had been working closely together to find a way for family and friends to see each other, recognising it must be “limited and cautious”.

Each Christmas bubble can meet at home, at a place of worship or an outdoor public location, but existing, more restrictive rules on hospitality and other venues will be maintained throughout the period.

Andrew Hayward, professor of infectious diseases epidemiology at University College London, and a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), warned easing measures would lead to increased transmission and a possible “third wave” of infection.

“Effectively what this will be doing is throwing fuel on the Covid fire,” the professor, who is also a Sage member, told BBC2’s Newsnight on Tuesday.

He added: “With the vaccine on the way, if we are not very careful over Christmas we are really in danger of snatching defeat from the jaws of victory on this one.”

The Christmas plan was agreed following a Cobra meeting chaired by Cabinet minister Michael Gove bringing together the Westminster Government and the devolved administrations.

Despite the new measures, families and groups of friends will still face difficult decisions and restrictions on their activities.

The bubbles will have to be exclusive over the five-day period, meaning people cannot shift from one group to another – although children whose parents are separated will be allowed to move between them.

People aged over 65 in care homes will not be able to join their loved ones for Christmas, and in families where three children live away from home, they would not all be able to return for the festive period.

Government figures showed a further 608 people had died within 28 days of testing positive for Covid-19 as of Tuesday, bringing the UK total to 55,838.

Meanwhile, a further 11,299 lab-confirmed cases of coronavirus were reported on Tuesday, the lowest figure since early October.