Two leading medical journals are calling on the government to abandon its rash plan to allow household mixing over Christmas.
The British Medical Journal and Health Services Journal said the temporary lifting of restrictions might be viewed by people "as permission to drop their guard", risking a third wave of coronavirus.
The warning comes as millions of people in London and other parts of south-east England will move to the toughest coronavirus restrictions on Wednesday.
There are fears the five days of relaxed rules over Christmas will make the situation worse.
The UK government is also caught up in a battle with parents and local authorities over whether schools should close early for the holidays.
Addressing the Christmas plan, the journals wrote in a joint editorial: "Members of the public can and should mitigate the effect of the third wave by being as careful as possible over the next few months. But many will see the lifting of restrictions over Christmas as permission to drop their guard. The government was too slow to introduce restrictions in the spring and again in the autumn.
"It should now reverse its rash decision to allow household mixing and instead extend the tiers over the five-day Christmas period.
"In order to bring numbers down in advance of a likely third wave, it should also review and strengthen the tier structure, which has failed to suppress rates of infection and hospital admission."
It is only the second joint editorial in the more than 100-year histories of the medical journals.
"We are publishing it because we believe the government is about to blunder into another major error that will cost many lives," they said.
The warning is likely to cause further confusion in London and other parts of south-east England that go into the toughest Tier 3 coronavirus restrictions from Wednesday, before rules are eased over the Christmas period, with fears that this will make the situation worse.
The UK government is still caught up in a battle with parents and local authorities over whether schools should close early for the holidays, and people are being told it is acceptable to visit family at Christmas, but to do "the minimum".
Some parents are taking their children out of school early so they can see their families at Christmas, many of them worried they will be told to isolate and then be unable to mingle over the festive period.
Meanwhile, government scientists said a new strain of coronavirus may be linked to a resurgence in Covid-19 cases in south-east England.
Some local authorities took action independently of the government by closing schools early, while parents have resorted to taking their children out of classrooms in those that remain open.
London Mayor Sadiq Khan said Tier 3 rules would not be enough to curb the infection rate among those aged 10 to 19 – the age bracket where the virus is rising the fastest in the capital.
Mr Khan blamed the government for “retrofitting solutions based on an ineffective tiering system” rather than shutting schools.
He told the BBC Radio 4's Today programme: "The virus is spreading faster among our children. Tier 3 stops pubs and bars from opening but all the places where the virus can spread are still open.
“The particular problem we have in London is with the absence of community testing in schools. Many children, despite the heroic efforts of teachers, could well have the virus and not know about it.”
He said: "These very same children next week will be hugging and kissing granny because the rules have been relaxed.
“We’re going from Tier 2, to Tier 3, to Tier 0 and back to Tier 3, in advance of potentially another national lockdown in January.”
Chief Secretary to the Treasury Stephen Barclay said the government had no plans to tighten the Christmas rules, which allow up to three households to form a bubble and meet between December 23 and 27.
However, he urged caution by insisting families "do the minimum" when meeting up. He denied the government was "letting people loose" and insisted the Christmas window was not a Tier 0 situation.
He told Sky News: "It is important for people's well-being, for their mental health. We don't want to criminalise people for coming together as family over Christmas. But it is important that people do the minimum that is possible. So, people will be making their own judgments."
Mr Barclay said “all options” are open to the government in relation to councils that have decided to close schools early.
The government ordered Greenwich council, in London’s south-east, where schools closed on Monday, to reopen the schools on Tuesday.
Mayor Danny Thorpe said although he did not agree with the government’s advice he had been left with “no choice but to ask schools to keep their doors open”.
“With Covid-19 cases rising rapidly in the borough, I cannot agree that this is the correct choice for our schools. However, I also cannot justify the use of public funds to fight the decision in the courts," he said.
TTwo other London councils were also moving to online learning from Tuesday.
The Office for Standards in Education said England was in a "very difficult situation".
Head of Ofsted Amanda Spielman told the BBC: "It's so easy to call for closures and forget the long-term price that children pay, which our visits show so clearly.
"We've had children yo-yoing in and out of school through the autumn and really suffering as a result. We need clarity, consistency, not last-minute decisions.”
