British officials have designated a coronavirus strain first found in India as a "variant of concern" after a cluster of cases were discovered in England.
Scientists from Public Health England believe the variant known as B.1.617.2 spreads more quickly than the original version of the virus.
PHE also said B.1.617.2 seems to be as transmissible as the Kent variant that fuelled much of Britain's second Covid-19 wave, but there is no evidence it is resistant to current vaccines.
In an official statement on Friday, PHE confirmed that it is now monitoring the strain closely as a Variant of Concern (VOC) and said its other characteristics are still being investigated.
Confirmed cases of B.1.617.2 increased to 520 from 202 over the last week, with most found in people who had gone abroad or had contact with a traveller.
"There is currently insufficient evidence to indicate that any of the variants recently detected in India cause more severe disease or render the vaccines currently deployed any less effective," PHE said.
Dr Deepti Gurdasani, a clinical epidemiologist and senior lecturer at Queen Mary University of London, said the B.1.617.2 strain could become the dominant strain in London by May if infection numbers continue to climb.
"At the current doubling rate [B.1.617.2] could easily become dominant in London by the end of May or early June", she told The Guardian.
B.1.617.2: What other variants of concern are there?
Other variants of concern include strains first identified in Kent, south-east England, as well as South Africa and Brazil.
British Prime Minister Boris Johnson said on Friday the government needed to handle carefully the emergence of new strains.
His comments were made as it was announced on Friday that the estimated reproduction, or "R", rate in England dipped to between 0.8 and 1, but the epidemic might be shrinking a little less quickly than previously, Britain's health ministry said. This means that, on average, every 10 people infected will infect between eight and 10 other people.
"I think we've got to be very careful about that. We're doing a huge amount, obviously, to make sure that when we do find outbreaks of the Indian variant that we do surge testing, that we do door-to-door testing," Mr Johnson told reporters.
The variant is believed to be fuelling the deadly wave of coronavirus in India, which on Friday reported another day of record infections, with 414,188 confirmed cases in the past 24 hours. The Indian Health Ministry reported 3,915 deaths, bringing the country's estimated total to 234,083.
Covid-19 still a threat to UK
Experts say the medium-term threat from the virus has not gone away, despite successful vaccine campaigns in developed nations.
England’s chief medical officer Prof Chris Whitty said the world would continue to record a significant number of deaths unless more effort was made to vaccinate the vulnerable.
He said that while coronavirus may become a “much milder, chronic disease” in the long term, new variants would continue to cause problems.
“In the medium term, the outlook still looks pretty bleak around the world,” he told a Royal Society online event on Thursday.
“I would really reiterate that until we have got a situation where we have induced immunity in those who are most vulnerable everywhere in the world, we will continue to see really significant morbidity and mortality from this virus.”
Prof Whitty said that while time and science “was on our side”, the virus was "not going to go away”.
Prof Wendy Barclay, who is advising the UK government on its Covid-19 response, said there was “a lot to do” to bring the pandemic under control.
“We will not eradicate this virus,” she said.
“It is so far spread around the world and the vaccines do not necessarily completely prevent transmission. I think we will live with the derivatives of Sars-Cov-2 for a very long time.”