However, it has been identified in other parts of the country too, according to a committee document following a meeting on the new strain, which has been dubbed VUI-202012/01.
The new strain of coronavirus identified in England grew exponentially during the second national lockdown, government scientific advisors have said.
The New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) said the variant is concentrated in London, southeast England and the east of England – most of which were placed under a new tier 4 level of restrictions over the weekend.
NERVTAG said the variant showed “exponential growth” during the time of the second national lockdown, which ran from early November to early December.
“In summary, NERVTAG has moderate confidence that VUI-202012/01 demonstrates a substantial increase in transmissibility compared to other variants,” the document said.
A growing number of nations – including France, Italy, Germany, Finland and Denmark – have banned flights from the UK in a bid to stop a mutant strain of coronavirus crossing their borders.
NERVTAG said few cases of the variant have been reported in other countries around the world, but there was “one confirmed export” from the UK to Australia.
It was noted that other countries have lower sequencing capability than the UK, the committee document said.
On Monday, the French health minister said it was “entirely possible” the new mutant strain of coronavirus was circulating in France – although no cases of it have not been identified yet.
In Italy, authorities said the variant had been detected in a traveller who had recently come from the UK.
Over the weekend, millions of people in London and surrounding counties have been placed in tougher restrictions and told they cannot meet with people outside their household for Christmas, while the festive relaxations were slashed to just one day for the rest of England.
“Given the early evidence we have on this new variant of the virus and the potential risk it poses, it is with a very heavy heart I must tell you we cannot continue with Christmas as planned,” Boris Johnson, the UK prime minister, told the nation.
On Saturday, Mr Johnson said there was “no evidence” the new variant “causes more severe illness or higher mortality” but “it does appear to be passed on significantly more easily”.
He said the new strain could be up to 70 per cent more transmissible than the old virus variant.
The PM will hold crisis talks with ministers on Monday after France banned lorries carrying freight from the UK and the flight bans taken by countries around the world in response to the new variant.