Top UK scientists say Boris Johnson’s plan to relax coronavirus restrictions at Christmas is “throwing fuel on the Covid fire” and could cause a third wave of the pandemic.

The PM announced on Tuesday that three households will be able to mix in a bubble from December 23 to 27.

Johnson said the agreement between his government and devolved administrations meant families would be able to come together across the UK, but added: “The virus doesn’t know it’s Christmas and we must all be careful.”

Reacting to the news, a member of the government’s Scientific Advisory Group for Emergencies (Sage) warned last night the planned relaxed restrictions could lead to a third wave of the pandemic.

Professor Andrew Hayward told BBC2’s Newsnight: “Effectively what this will be doing is throwing fuel on the Covid fire.

“I think it will definitely lead to increased transmission. It is likely to lead to a third wave of infection, with hospitals being overrun, and more unnecessary deaths.

“We are still in a country where we have got high levels of infection with Covid, particularly in young people. Bringing them together for hours, let alone days, with elderly relatives, I think, is a recipe for regret for many families.”

Hayward added that with vaccines on the way “we are really in danger of snatching defeat from the jaws of victory on this one”.

The sentiments were echoed by colleagues in the scientific community. Professor Graham Medley, an expert in infectious disease modelling at the London School of Hygiene and Tropical Medicine, said the move could lead to more people being admitted to hospital and further lockdown measures in the New Year.

The Sage attendee told BBC Radio 4’s Today programme: “I think we’re in a process now whereby the population’s risk of filling up the NHS is really being passed down to us as individuals.

“For other diseases like flu or hepatitis B, the government doesn’t get involved in helping or determining what our risk is and it is really, I think, for this Christmas up to us as individuals and families to think about what our risks are and how we are going to mitigate them.

“I think it is inevitable that if a lot of people do take that risk, even if it is a small risk, then we will end up with a lot of people in hospital and potentially having to take measures in January to lock down again.”

Prof Medley advised people to isolate before visiting relatives, to consider the amount of time they plan to spend with them, to remain “completely faithful” to any social bubble arrangements and to weigh up the risk of spreading Covid to those who are vulnerable.

But he said, even with mitigations in place, social interactions come with risks that “could play out very badly for some people”.

Paul Hunter, professor of medicine at the University of East Anglia, had a slightly more positive take, noting the festivities may be what is needed to “make it through the rest of winter”.

But he still cautioned that the relaxation of restrictions will “almost inevitably” lead to an increase in transmissions.

“The issue is whether that increased risk is tolerable in relation to the benefits. At that time schools will be closed so there would naturally be some downward pressure on transmission,” he said.

“Also if the new tier system is working well and local authorities are placed into a more appropriate tier this time around then there will be a downward pressure on transmission before and after the Christmas break.”

He added: “Providing that the new tier system is better managed than in October any increase in cases could be relatively short lived. After Christmas we will still have to live through a few more months of restrictions at least.

“Christmas whether or not we celebrate the day as a religious festival may be what we need to make it through the rest of winter.”