Britain identified more than 1,000 people who were infected with a new fast-spreading Covid-19 variant, predominantly in the south of the country where it could be connected to a surge in cases.
Health Secretary Matt Hancock said on Monday that the World Health Organisation had been notified of the scientific findings.
"We have identified a new variant of coronavirus, which may be associated with the faster spread in the south-east of England," Mr Hancock told parliament.
"Initial analysis suggests that this variant is growing faster than the existing variants," he said, as he announced that London and other regions would be placed under the toughest Tier 3 restrictions from Wednesday.
The WHO emergencies expert Mike Ryan said there was no evidence yet that the strain behaved differently to existing types of the virus.
"We are aware of this genetic variant reported in 1,000 individuals in England," he said in Geneva. "Authorities are looking at its significance. We have seen many variants, this virus evolves and changes over time."
Mr Hancock said it was highly unlikely that the vaccines currently being developed would not work with the new strain.
The original strain, detected in China's Wuhan city in December 2019, is the L strain. The virus mutated into the S strain at the beginning of 2020. That was followed by V and G strains. Strain G mutated yet further into strains GR, GH and GV. Several other infrequent mutations were collectively grouped together as strain O.
Mr Hancock said the new variant was being cultured at the British Army's Porton Down medical and bioscience laboratories.
G strains are now dominant around the world. One specific mutation, D614G, has become the most common variant.
The most recent mutation to emerge is the GV strain, which has so far been isolated to Europe. Experts there say it is unclear whether the strain is spreading because of any transmission advantage or because it affected socially active young adults and tourists over the summer.
The Covid-19 virus has so far mutated slowly and scientists have been divided on the implications of some of the mutations.
Some experts said that the D614G variation made the virus more transmissible, but other studies contradict that.
The changes so far have not resulted in strains that were likely to be resistant to vaccines in development.
However, experts who have watched influenza and HIV mutate over the years, evading vaccines, say that future mutations of SARS-CoV-2 remain unknown.
The best chance of avoiding changes that make the virus impervious to a vaccine remains curtailing its spread and reducing the opportunities it has to mutate.