England's chief medical officer has confirmed that a new mutant Covid-19 strain spreads more rapidly - and urgent work is underway to see if it is more deadly.
Prof Chris Whitty said the UK has informed the World Health Organisation about the new variant.
At the moment he said there is "no current evidence" to suggest the new strain causes a higher mortality rate or affects vaccines, but work is being carried out to confirm this.
His statement comes ahead of his appearance at a Downing Street briefing at 4pm alongside Boris Johnson and Sir Patrick Vallance, England's chief scientific officer.
Prof Whitty said: "As announced on Monday, the UK has identified a new variant of Covid-19 through Public Health England's genomic surveillance.
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"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 World Health Organisation and are continuing to analyse the available data to improve our understanding."
The chief medical officer continued: "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.
"Given this latest development, it is now more vital than ever that the public continue to take action in their area to reduce transmission."
The PM today called an emergency cabinet meeting before addressing the public.
The Prime Minister has come under intense pressure this week over his 'Christmas bubbles' as Covid-19 rates soar in parts of the UK, with the country said to be on a "knife edge".
Ministers met yesterday to decide how the mutant strain, which has ripped through London, Kent and Essex, could be controlled.
Medics warn that the new strain is 50% more contagious than previous Covid-19 cases.
Ministers are understood to have discussed a possible ban on commuters entering London and travel to and from the South East.
Mr Johnson said yesterday he hoped that England would not need to go into a third lockdown after Christmas, but scientists have warned that urgent action might be needed to curb the spread of the virus.
The UK reported 28,507 new Covid-19 cases yesterday and 489 deaths.
