Professor Andrew Hayward said the planned move to ease measures over the festive period amounts to “throwing fuel on the Covid fire”.
“I think it will definitely lead to increased transmission,” he told BBC2’s Newsnight on Tuesday.
“It is likely to lead to a third wave of infection, with hospitals being overrun, and more unnecessary deaths.”
UK governments unveil Covid-19 Christmas plans
“We are still in a country where we have got high levels of infection with Covid, particularly in young people,” Prof Hayward added.
He described the decision to allow people to meet in bubbles at Christmas as a “recipe for regret for many families.”
“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,” he said.
The UK Government and devolved administrations have agreed a temporary easing of measures which will allow three households to mix in a bubble from December 23 to 27.
Social distancing will be relaxed within the bubbles, giving people the chance to hug friends and family for the first time in months.
Boris Johnson outlines UK Christmas agreement
BMA UK council chair Dr Chaand Nagpaul said: “There is a careful balance to be struck when weighing up the risks associated with Covid-19 and the understandable wish to see loved ones this Christmas.
“This virus does not discriminate against certain days of the year.
“Relaxing the rules on indoor mixing for a five-day period will almost certainly carry the risk of a rise in infection rate and possibly more hospitalisation and deaths, adding further pressure on the health service, doctors and NHS staff.
“With infections levels and hospitalisations still worryingly high, and the daily death toll in the second wave now rising, we do not want loved ones to become seriously ill, hospitalised or lives put at risk this Christmas.
“The priority now must be to support the public to adhere to stringent rules around physical distancing and infection control to drive down the infection rates further by Christmas. The lower the level of infection the less risk it will place for families to meet at Christmas.”
Dr Nagpaul added that “it is absolutely vital” people adopt the necessary safety precautions if mixing with other households, such as ventilating rooms and limiting physical contact when masks are not worn.
Meanwhile Paul Hunter, professor of medicine at the University of East Anglia, said Christmas might be what is needed to “make it through the rest of winter”.
He said: “Any relaxation of the restrictions over the Christmas period will almost inevitably lead to some increase in transmission, and therefore illness, hospitalisations and sadly deaths.
“The issue is whether that increased risk is tolerable in relation to the benefits.”
Prof Hunter said there will be “some downward pressure” on transmission due to schools being closed for the Christmas break, while there could be a similar effect from the tier system “working well”.
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.”