Ministers have met to discuss how to contain the rising number of coronavirus infections in England, reports BBC.
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.
Analysis suggests the R number - which represents how many people each infected person passes the virus onto - has risen above 1 in the UK.
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.
Ewan Birney, deputy director general of European Molecular Biological Laboratory, said the new 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.
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Ministers met on Friday to discuss what action will be necessary to deal with it, 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.
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.
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."
