Ministers have met to discuss how to contain the rising number of coronavirus infections in England.
It comes as Prime Minister Boris Johnson said he was "hoping to avoid" another national lockdown in England.
Former Health Secretary Jeremy Hunt said the government needed "to respond to what is happening on the ground" with hospital admissions rising.
Meanwhile, millions more people in southern England are now living under the toughest Covid restrictions.
Government scientists are continuing to evaluate the spread of a new variant of Covid in south-east England as there are "growing concerns" about its transmission.
Hospitals in Kent are postponing non-urgent procedures as coronavirus cases in the county rise beyond figures seen in the spring.
Ewan Birney, deputy director general of European Molecular Biological Laboratory, said the new Covid variant had been growing "very strongly in the south of England" but it was not possible to say definitively that it was transmitting faster than others or whether it was because the number of cases in general was growing.
However, he added most scientists "think it is going faster - that it really is a property of the virus".
He told BBC Radio 4's Today programme: "I must stress how complicated it is to work out, in a situation where things might be growing for other reasons, to really put your finger on that it's actually the virus which is doing it but the evidence is pointing in that direction."
On Monday, Health Secretary Matt Hancock said the variant may be associated with the faster transmission of the virus in the South East but there was "nothing to suggest" it caused worse disease or that vaccines would no longer work.
Dr Birney said it was too soon to know whether the variant caused worse disease but scientists should start to get a good idea "in a matter of weeks", adding that other viruses had tended to mutate to become faster transmitters but less harmful.
Ministers met on Friday to discuss what action will be necessary to deal with the new variant, but a Downing Street source said the government is "not there yet" on rethinking Christmas plans.
The decision by all four UK nations to relax restrictions and allow more household mixing for five days over Christmas has prompted concerns about a further surge in case numbers.
Analysis suggests the R number - which represents how many people each infected person passes the virus onto - has risen above 1 in the UK.
Northern Ireland and Wales have both announced post-Christmas lockdowns, while the Scottish government has said "all options" remain on the table ahead of a review on Tuesday.
'A changing situation'
Health bosses have warned the NHS is under significant pressure, with nearly 90% of hospital beds in England full.
Mr Hunt, chair of Parliament's health select committee, told the Today programme that the current situation was "very serious" and if the government did change its mind about relaxing the rules "we should certainly not condemn it as a screeching U-turn but the responsible thing to do in a pandemic when the facts change".
He cited two big developments - the new Covid variant and hospital admissions "going up very, very sharply" - adding that "we have to look at the changing situation".
If ministers did not want to change the law, Mr Hunt said they should consider strengthening the guidance on social distancing, adding: "It would be an enormous tragedy if we had a spike in deaths at the end of January/February because we took our foot off the pedal this close to having a vaccine."
Asked about reports that the Oxford-AstraZeneca vaccine could be approved for use in the UK by the end of the year, Mr Hunt said that would make "a massive difference".
He said the UK had doses of the Pfizer vaccine to "keep us going until the end of January" but then there wouldn't be another shipment until March so having the Oxford vaccine in January would mean the UK could "keep the rollout going at its current pace".
A spokeswoman for the UK regulator MHRA said its review of the vaccine was "ongoing".
Covid rules are due to be relaxed across the UK between 23 and 27 December, with up to three households being able to meet.
But the prime minister has urged people to think about elderly relatives to "avoid spreading the disease" over the holiday period.
Labour leader Sir Keir Starmer has called for Mr Johnson to "toughen up over Christmas", saying the Welsh government's decision to limit Christmas bubbles to two households - instead of three - was a "step in the right direction".
Leon Danon - an epidemiologist who sits on the Scientific Pandemic Influenza Group on Modelling which advises the government - has been trying to model the impact on the R number of various households getting together at Christmas.
He told Today that modelling showed putting three households together had a "pretty bad" effect on the R value, however he said over the festive period this would be counterbalanced by schools closing and fewer people going to work reducing people's other contacts.
Meanwhile, tier three Covid rules have come into force for parts of southern England, meaning that 38 million people - more than two-thirds of the nation's population - are now subject to the toughest restrictions.
The changes, which came into effect at 00:01 GMT, see Bedfordshire, Buckinghamshire, Berkshire, Hertfordshire and parts of Surrey, East Sussex, Cambridgeshire and Hampshire join the list of areas now in the highest level of England's three-tier system.
In tier three, pubs and restaurants must close and different households cannot mix indoors or in most outdoor venues.
On Friday, the UK recorded a further 28,507 cases, along with 489 deaths within 28 days of a positive test.
Official figures show Covid-19 cases have risen in the past week in England, driven by sharp increases in London, as well as rises in the South East and East Midlands.
Average NHS bed occupancy in England has reached almost 89% for the week ending 13 December, with 59 out of 126 NHS trusts reporting bed occupancy of higher than 90% - which is above the recommended safe level.
The R number is estimated to be between 1.1 and 1.2, up from between 0.9 and 1 last week.
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