The variant of coronavirus first identified in India, known as Delta, has become the "dominant" strain in Britain, Public Health England (PHE) said Thursday, Trend reports citing xinhua.
The number of cases of the Delta variant has risen by more than 5,000 since last week to 12,431, according to the data from PHE released Thursday.
PHE officials say that the Delta variant has now overtaken the Kent variant, known as Alpha, as the most dominant in Britain.
Dr Jenny Harries, chief executive of the UK Health Security Agency said: "With this variant now dominant across the UK, it remains vital that we all continue to exercise as much caution as possible."
"The way to tackle variants is to tackle the transmission of COVID-19 as a whole. Work from home where you can, and practice 'hands, face, space, fresh air' at all times," Harries said.
There is "early evidence to suggest there may be increased risk of hospitalisation" from Delta, but further data is needed to confirm this, according to PHE.
Earlier on Thursday, British Transport Secretary Grant Shapps announced that Portugal will be moved from the green to amber travel list, which will require people returning to quarantine for 10 days.
The move was taken due to concerns about the Nepal mutation of the Delta variant, said Shapps.
As a result, Portugal's positivity rate has "nearly doubled since the last review", he said.
British Health Secretary Matt Hancock confirmed Wednesday that more than three-quarters of adults in Britain have now had their first COVID-19 vaccine dose.
More than 39.5 million people have been given the first jab of the coronavirus vaccine, according to the latest official figures.
The British government is facing increasing pressure to delay the final step of unlocking restrictions in England on June 21 due to concerns over the spread of the Delta variant.
From May 17, pubs, bars and restaurants in England were permitted to open indoors, while indoor entertainment resumed, including cinemas, museums and children's play areas.
People were also allowed to travel abroad to a number of green-list countries without having to quarantine upon return as the ban on foreign travel has also been lifted.
The British government's roadmap is expected to see all legal limits on social contact to be removed on June 21. It is understood that a final decision on the planed easing of lockdown will not be made until June 14.
Experts have warned that coronavirus may continue to evolve for years to come, and eventually it is likely current vaccines will fail to protect against transmission, infection, or even against disease caused by newer variants.
To bring life back to normal, countries such as Britain, China, Russia, the United States as well as the European Union have been racing against time to roll out coronavirus vaccines.