A fourth tier of coronavirus restrictions is expected to be introduced in London and south-east England, sources have told the BBC.
There is also expected to be a tightening of the plans to relax the rules around households gathering during the Christmas period.
The PM is to hold a press conference at 16:00 GMT as concerns grow about the spread of a new variant of the virus.
Boris Johnson hosted a cabinet meeting earlier to discuss what action to take.
Chief medical officer Prof Chris Whitty said government advisers “consider the new strain can spread more quickly”.
He will join the prime minister and Sir Patrick Vallance, the government’s chief scientific adviser, at the press conference.
England’s current three-tier system of coronavirus measures has faced criticism for not being effective in stopping the surge in cases.
The first ministers from Scotland, Wales and Northern Ireland held talks with cabinet minister Michael Gove earlier.
Scotland’s Nicola Sturgeon, who will hold a press conference later, tweeted: “Cases currently at lower level in Scotland than UK – but preventative action may be necessary to stop faster spreading strain taking hold.”
Welsh ministers are meeting to discuss “serious concerns”, while Northern Ireland First Minister Arlene Foster tweeted that the new variant of coronavirus prevalent in south-east England was “very concerning”.
Wales and Northern Ireland have already announced post-Christmas lockdowns. Covid rules were due to be relaxed across the UK between 23 and 27 December, with up to three households being able to meet.
It was just 72 hours ago that Boris Johnson was resisting pressure to cancel relaxations of restrictions over Christmas. He told a press conference cancelling the festive plans would be “inhuman”.
But concern has grown significantly in the last couple of days; both about the rising number of cases and the new variant of the virus disclosed earlier this week.
Ministers met with their scientific advisers on Friday evening – and again on Saturday morning. Things have moved quickly in the last 24 hours.
Sources have confirmed a new tier 4 will be introduced in London and the South East of England. That’s likely to mean closure of non-essential shops and beauty salons.
That’s likely to have a significant impact of Christmas plans and what people can do in and around the capital.
There are also conversations taking place in the devolved administrations about whether further restrictions are needed there too.
Following concern about the rapid spread of a new variant, Prof Whitty said: “We have alerted the World Health Organisation and are continuing to analyse the available data to improve our understanding.”
That came after advice from the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) – the expert committee that advises the government on pandemics.
“There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments although urgent work is under way to confirm this,” Prof Whitty added.
Chairman of Nervtag, Peter Horby, said they had not found the variant to be more severe but being easier to transmit meant it was harder to control.
Paul Hunter, professor of medicine at the University of East Anglia, said it was a big concern that the new variant seemed to be “out-competing the other viruses very fast indeed”.
He told the BBC News Channel: “The really worrying thing from my perspective is that this was something we didn’t know about at the beginning of this month, and it appeared and from what we hear, it is already becoming one of the dominant strains.”
Asked if the transmission would move northwards, he said “almost certainly”.
Viruses do mutate – so this development should not come as a complete surprise.
There is nothing to suggest it causes more serious illness or would impact the ability of the vaccines to work.
But preliminary investigation suggests it is leading to faster transmission.
That clearly is causing concern, especially ahead of Christmas when relaxing restrictions mean there is more opportunity for the virus to spread.
It may well explain why infection rates started increasing in London during lockdown – something that has baffled experts.
All indications are that the PM will issue new advice on Christmas.
Hospital admissions are increasing – with the number of available beds in parts of south-east England falling almost by the day.
Shadow health secretary Jonathan Ashworth called on the prime minister to set out “what action he will be taking” as the virus was “out of control in parts of the country”.
Jeremy Hunt, a former health secretary, said the government needed “to respond to what is happening on the ground” with hospital admissions rising.
Health bosses have warned the NHS is under significant pressure, with nearly 90% of hospital beds in England full.
Hospitals in Kent are postponing non-urgent procedures as coronavirus cases in the county rise beyond figures seen in the spring.
Analysis suggests the R number – which represents how many people each infected person passes the virus onto – has risen above 1 in the UK.
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.