A new strain of coronavirus may be causing new infections in parts of the UK, it was announced today.
Health Secretary Matt Hancock revealed the discovery of the mutation, which could be an even more contagious form of the Covid-19 virus.
It was announced that London and the surrounding area would be placed under Tier 3, the UK's strictest level of coronavirus restrictions.
Here's what we know about the new Covid strain so far.
What is the new Covid strain?
Little is known about the Sars-CoV-2 mutation, but the discovery has fuelled fears that it could be responsible for a surge in Covid cases.
More than 1,000 people are believed to have been infected with the fast-spreading strain in the south-east of England.
Earlier today, Mr Hancock said London's new Tier-3 measures would come into effect on Wednesday.
He said the discovery of the strain meant the government had to take "swift action".
"Over the last week, we've seen very sharp, exponential rises in the virus across London, Kent, parts of Essex and Hertfordshire," Mr Hancock told the House of Commons.
"We do not know the extent to which this is because of the new variant but no matter its cause, we have to take swift and decisive action," he said.
Will vaccines work effectively against the new strain?
The changes so far have not resulted in strains that are likely to be resistant to vaccines in development.
But experts who have watched influenza and HIV mutate over years, evading vaccines, warn future mutations of the coronavirus remain unknown.
Does the new strain bypass masks and washing hands?
The WHO emergencies expert Mike Ryan said there was no evidence yet that the strain behaved differently to existing types.
"We are aware of this genetic variant reported in 1,000 individuals in England," Dr Ryan said in Geneva.
"Authorities are looking at its significance. We have seen many variants. This virus evolves and changes over time."
Officials in the UK say washing hands, wearing masks and keeping distance remain the best way to stop the spread of the virus.
Is it more infectious?
Scientists are not yet sure if this is the case.
Experts say socially active young adults and tourists may have been responsible for the rise over summer.
It had been suggested that an earlier mutation, D614G, made the virus more transmissible, but other studies contradict that claim.
What are the other strains called?
The original strain, detected in China's Wuhan city in December 2019, is the L strain.
Strain G mutated further into strains GR, GH and GV, and are now the most common variants.
The most recent mutation to emerge is the GV strain, which has so far been isolated to Europe.