Boris Johnson and top scientific advisers will hold a Downing Street press conference at 4pm on Saturday amid reports he will announce new stricter rules for Christmas in England.
The prime minister is speaking to Cabinet ministers after the alarm was raised over a sharp spike in Covid infections, particularly in London and the south east.
It is reported that the recently-identified new variant of coronavirus may be more infectious and there is evidence that a third wave is building.
Whitty said the government had alerted the World Health Organisation (WHO) over the new variant’s “rapid spread” and underlined it was “more vital than ever” the public take precautions.
The news comes as families prepare for Christmas, with a five-day relaxation of restrictions still planned for December 23 to 27.
Health secretary Matt Hancock said last week that the virus had doubled in the space of seven days in some parts of the south east.
Whitty said in a statement: “As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the South East, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now consider that the new strain can spread more quickly.
“We have alerted the WHO and are continuing to analyse the available data to improve our understanding.
“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 underway to confirm this.
“Given this latest development it is now more vital than ever that the public continue to take action in their area to reduce transmission.”
Scotland’s first minister Nicola Sturgeon, meanwhile, has said “preventative action may be necessary” to halt the spread of the variant strain of the virus.
She tweeted: “Following a four nations Covid call earlier, I’ll chair @scotgov Cabinet meeting this afternoon to discuss emerging evidence on new variant.
“Cases currently at lower level in Scotland than UK – but preventative action may be necessary to stop faster spreading strain taking hold.”
Wales’ first minister Mark Drakeford said he would chair a meeting of his cabinet later.
He tweeted: “I attended a four nation call earlier today to discuss serious concerns over the spread of a new Covid variant. I will chair a meeting of the @WelshGovernment cabinet at 2pm today.”
On Friday, the R rate jumped above the crucial number 1 and is now 1.1-1.2, having been 0.9-1 the previous week.
Shadow health secretary Jonathan Ashworth has called for the government to set out a plan.
“This is an incredibly serious moment and people are worried. The prime minister has refused to take action to limit the virus’ spread over Christmas. All he has offered so far is confusion and indecision,” he said.
“He must now address the nation to explain what action he will be taking.”