The contagious UK variant of Covid-19 has mutated again, in worrying genetic changes that may give the virus the ability to evade the immune system, according to scientists.
Public Health England reported that tests on some samples detected genomes with the E484K mutation, which has already been seen in the highly transmissible South African and Brazilian variants.
Calum Semple, a top scientist advising the UK government, told BBC radio that this "mutation of most concern" had developed spontaneously in the UK variant first detected in Kent in England's south.
Sky's science correspondent Thomas Moore said the "worrying development" meant the virus had effectively "developed a superpower" that enabled it to not only infect cells, but also to evade the immune system.
Research suggests the mutation may be more resistant to antibodies, although Moderna and Pfizer have claimed their vaccines are still effective.
Early testing by Moderna found its vaccine could protect against the mutation, although the effects may not be as strong or last as long. AstraZeneca is due to announce results on its vaccine's effectiveness against the mutation this week.
UK Health Secretary Matt Hancock on Tuesday said a further 11 cases of mutations of concern had been identified in Bristol and 32 in Liverpool, warning of "renewed challenges posed by new variants of the coronavirus".
At least 105 cases of the variant first identified in South Africa have already emerged in the UK, including 11 with no links to international travel: in parts of London, the West Midlands, east, southeast and northwest England.
Authorities in these areas have started offering door-to-door and mobile testing as part of a push to test 80,000 people for the variant, and are sequencing every positive test.
"In all these areas it is imperative that people stay at home and only leave home where it is absolutely essential," Hancock said.
The Government is offered testing to everyone in these areas aged over 16, even if they have had the vaccine. Hancock urged all residents and everyone who has to leave home for essential work to get tested.
Workplace testing is also being expanded across the country, including at the UK parliament, and "more scientific work is under way to learn more about new variants", the health secretary said.
He said "the message is more important than ever to stay at home, maintain social distancing and get tested".
The UK has now vaccinated 9.2 million people, including a million in one weekend, he reported.
"We must all keep at it, we've sacrificed so much," he added, warning of "difficult weeks ahead".
The UK is ahead of many countries in the vaccine race, and has now given shots to 13.7 per cent of its population, compared with the US at 32.2 million or 7.9 per cent.
Israel leads the pack, with 37 per cent of its population having received at least one dose, and more than a fifth have already had a second.
But the European Union commission chief defended the slower Covid-19 vaccine in the EU, telling French newspaper Le Monde: "The commission and the member states agreed not to compromise on the safety and effectiveness requirements linked to the authorisation of a vaccine."