Professor Chris Whitty said it was right that loved-ones should be able to spend time together over the festive period, but urged caution among vulnerable relatives.
Speaking alongside Boris Johnson and chief scientific officer Sir Patrick Valance, Prof Whitty told the public to take the pandemic “really seriously” over the holidays.
“Don’t do stupid things,” he said. “Don’t do unnecessary things just because the rules say you can.”
Meanwhile, Sir Patrick encouraged people to keep their homes well-ventilated, socially distance where possible and “avoid behaviours that would spread the disease”.
He said: “I think hugging elderly relatives is not something to go out and do, it will increase the spread to a vulnerable population.”
Heath Secretary Matt Hancock has previously appeared to rule out hugging relatives and friends at Christmas.
Speaking on Times Radio earlier this month, he said: “I’ve got no doubt that people will continue to respect social distancing throughout, because we know that that is so important for full control of the virus.”
Under the plans, the Government pledged that relatives of care home residents in England will be able to hug their loved ones before Christmas if they test negative for Covid-19 and wear protective equipment.
The proposals, published on Monday, state: “The Government is committed, by Christmas, to providing twice-weekly testing to enable all care home residents to have regular visits from up to two visitors.
“If a visitor has a negative test, is wearing appropriate Personal Protective Equipment (PPE), and follows other infection control measures, then it will be possible for visitors to have physical contact with their loved one, such as providing personal care, holding hands and hugging.”
Boris Johnson outlines UK Christmas agreement
However, the Prime Minister and leading scintists have instructed the public to “think carefully” over the festive period.
Peter Openshaw, professor of experimental medicine at Imperial College London, said: “I do hope that people will not let caution evaporate over the Christmas period and think that a short break from restrictions will be fine. It really won’t.
“I urge everyone to maintain precautions and to be inventive and ingenious in devising ways to meet up in safety.
“Close personal contact is the main way this virus transmits and it should be possible to celebrate without letting the virus spread.
“Meet outdoors if possible, wear masks if you can’t maintain safe distance, wash your hands and use gel.
“Feeling well does not guarantee that you don’t have the virus: kissing your grandparents may be delivering a deadly dose of virus. Be pleased to see them but keep a safe distance.”
Dr Julian Tang, clinical virologist at the University of Leicester, said: “There is a risk of a third wave in January 2021 if we relax too much at Christmas.
“Most people in the country are still susceptible to the virus and any mixing will just give the virus a chance to spread further.
“For some, this year’s Christmas contact will be very important – perhaps more so than in other years.
“But if the rest of us who can tolerate a Zoom Christmas can do this to reduce the transmission risk for those who can’t, this will also reduce the virus spread for that bit longer, which will help all of us – and hopefully prevent a New Year third wave and possibly another national lockdown.”
Mr Johnson also warned any substantial “easing off” over the festive five-day period could lead to another national lockdown in the New Year.
From next Wednesday, more than 55 million people face tougher restrictions in England with only the Isle of Wight, Cornwall and the Isles of Scilly set to enter the lowest level, Tier 1.
Large swathes of the Midlands, North East and North West are in the most restrictive Tier 3, but the majority of people – including London – will be in Tier 2.
At Thursday’s press conference, Mr Johnson said: “I’m sorry to confirm that from Wednesday most of England will be in the top two tiers, with the toughest measures.
“And I know that this will bring a great deal of heartache and frustration, especially for our vital hospitality sector.”
But he warned: “If we ease off now we risk losing control over this virus all over again, casting aside our hard-won gains and forcing us back into a new year national lockdown with all the damage that would mean.”