The British Medical Association (BMA) said Prime Minister Boris Johnson should have tightened the planned relaxations between December 23 and 27 to protect the public.
The union urged people to make the “right choices” by keeping indoor socialising “to an absolute minimum” over the festive period and not necessarily form three-household bubbles despite being permitted to.
It also pleaded for people in England to follow the Tier rules for their area before and after the five-day Christmas window.
The BMA’s comments come after Mr Johnson used a Downing Street press conference on Wednesday to encourage the public to have a “smaller” and “safer” Christmas, urging them to “think hard” about their actions.
Original UK-wide plans for Christmas would have seen three households allowed to form a festive bubble between December 23 and 27, but Wales will now legislate to restrict this mixing to two households.
BMA council chairman Dr Chaand Nagpaul said: “Relaxing the rules will, without doubt, cost lives and the impact on the NHS in the new year will be grave.
“For those reasons, as the voice of thousands of doctors, we urge people to think long and hard about how many people they share their Christmas with.”
Dr Nagpaul added: “None of us would knowingly wish our loved ones to suffer or put their lives at risk and it is vital that we all take precautions to protect each other.
“The BMA believes the Prime Minister should have used today as an opportunity to restrict the relaxations in order to protect people in England – as we have seen the Government in Wales do.
“However, without this change, we as doctors are urging that people avoid meeting unless absolutely necessary.”
Dr Nagpaul said doctors were concerned about how the NHS would cope with another surge in infections, with hospitals “already struggling to cope” with “huge waiting lists”, including for patients with non-Covid conditions.
“The NHS has never been tested to this level and we simply don’t know how or if it will be able to cope if there is a third wave of infections,” Dr Nagpaul added.
Dr Rachel McCloy, associate professor in the Department of Psychology and Clinical Language Sciences at the University of Reading, said the updated Christmas guidance was “another example of quickly changing and ambiguous messages” from the Government.
She said: “At times when messaging has been mixed or ever-changing, public trust has been shaken and misinterpretations of the rules have been more common.”
She said a key question was whether the new tougher guidance should have accompanied the initial Christmas rules announcement as it was more closely aligned to expert medical advice at the time.
Dr McCloy added: “If the Government had ensured that the rules for meeting up over Christmas aligned closely with the guidelines for minimising infections, people in the UK would have been better supported in making safer decisions over the festive period.”