British scientists said relaxing coronavirus restrictions over Christmas "throws fuel on the fire” at a time when infections are high.
The warning comes as ministers consider giving families five days of freedom over the festive period.
The move would allow families to come together – bypassing a ban on people from several households meeting indoors.
But health officials said that a single day of freedom could cost five days of restrictions, raising the possibility of 25 days of tough measures in January.
Dr Susan Hopkins, a medical adviser to the government on its Covid-19 response, said she was “very keen” for the UK to have a near-normal Christmas but said that it would come at a price.
She said: “We need to be very careful about the number of contacts we have to reduce transmission before Christmas and get our cases as low as possible.”
Prof Andrew Haywood, who is on the government’s Scientific Advisory Group for Emergencies, said widespread mixing at Christmas posed “substantial risks” at the wrong time.
He told BBC's Radio 4 Today programme: "We're putting far too much emphasis on having a near-normal Christmas.
“We know respiratory infections peak in January, so throwing fuel on the fire at Christmas can only contribute to this.”
He said: “We’re on the cusp of being able to protect those elderly people who we love through vaccination and it would be tragic to throw that opportunity away and waste the gains we’ve made during lockdown.”
'Highly inconsistent message'
Prof Haywood also said the government’s communication strategies lacked consistency.
“When policy is undulating between stay at home to save lives, eat out to help out, the tier system, the second lockdown and now a proposal for an amnesty on social distancing, it’s a highly inconsistent message,” he said.
Another top scientist, Prof Gabriel Scally from Bristol University, said there was “no point” in having a near-normal Christmas only to have to “bury friends and relations in January and February”.
He tweeted: “We have not made nine months of sacrifices to throw it all away at Christmas.”
England is scheduled to emerge from its second coronavirus lockdown on December 2 and is expected to re-enter tier-based restrictions.
Much of the country is expected to fall into Tier 3 – or very high alert – which prevents people meeting indoors.
Defence Secretary Ben Wallace said a decision would be made as close as possible to December 2 but said that he did not want to be “the Grinch who stole Christmas”.
He told ITV's Good Morning Britain: "I would love all of us to be able to have a Christmas, but more than anything I want us to get through this Covid and try and get this country back to normal and I want to protect lives."
Meanwhile, Prof Andrew Pollard, of the Oxford Vaccine Group, said the world was “still at the bottom of the mountain” in the struggle to return to normal life.
His comments came after The Lancet medical journal published encouraging results from the Oxford trial that showed their vaccine triggered a strong immune response in adults in their 60s and 70s.
“Now we’ve got to get the data about the vaccine in front of regulators for them to scrutinise it and approve the first vaccines,” Prof Pollard said.
“Then we’ve got that huge effort to climb up to the top where we’ve got the vast majority of those at risk vaccinated and protected so the most vulnerable are no longer at risk and we can start to get back to normal.”