The highly infectious Covid variant first discovered in India was on Friday designated as a variant of concern (VOC) after cases more than doubled in a week, health chiefs announced.
Public Health England updated the designation of the the B.1.617.2 variant after the caseload increased to 520 from 202. Officials said almost half of the cases are related to travel or contact with a traveller.
Thought to be "at least" as transmissible as the B.1.1.7 Kent variant, which has rampaged around the world since November, cases of the new VOC have spread across the country. The majority of cases have been found in the North-West, predominantly in Bolton, and in London.
Cases of the variant, also known as VOC-21APR-02, have been reported in care homes and schools, according to leaked PHE documents seen by The Guardian.
The agency said there is currently insufficient evidence to indicate that any of the variants recently detected in India cause more severe disease or render current vaccines less effective.
Boris Johnson said officials were being "absolutely ruthless" in tracking down cases of the new variant.
"We're doing a huge amount, obviously, to make sure that where we do find outbreaks of the Indian variant, that we do surge testing, that we do door-to-door testing and isolate it, and also have very tough controls on transport and on people coming in," he added.
"At the moment we're looking carefully at the way the Indian variant seems to function. We don't see any evidence that it is resistant to the vaccines or in any way more dangerous."
Surge testing is being deployed in the affected neighbourhoods and enhanced genomic sequencing will take place.
Dr Susan Hopkins, the Covid-19 Strategic Response Director at PHE, said: "We are monitoring all of these variants extremely closely and have taken the decision to classify this as a variant of concern because the indications are that this VOC-21APR-02 is a more transmissible variant.
"The way to limit the spread of all variants is the same and, although we are all enjoying slightly more freedom, the virus is still with us. Keep your distance, wash your hands regularly and thoroughly, cover your nose and mouth when inside and keep buildings well ventilated and meet people from other households outside.
"If you are told to get a test, if you have any symptoms at all or have been in contact with someone who has tested positive, please make sure you get tested too."
Earlier this week Yasmin Qureshi, the MP for Bolton, said residents had been slow to get vaccinated. "People were a bit reluctant in whether to have the vaccine in Bolton,” she told the Manchester Evening News. "I've been working hard in trying to get people to take it. I'm hoping the vaccine process will continue and we get as many people vaccinated as fast as we can."