Urgent testing of 80,000 people for the South African strain of coronavirus is under way across England after authorities identified at least 11 cases with no known travel links.
A door-to-door testing programme will be run in parts of Surrey, southern England, where some cases were identified.
People in parts of London, east England, West Midlands and the North West are also being encouraged to take a test regardless of whether they have symptoms.
Previously, the UK had 105 cases of the virulent variant, but random checks identified 11 cases in the past week that could not be traced back to South Africa, suggesting community transmission.
Public Health England has been sequencing about five to 10 per cent of all positive Covid-19 cases to identify variants.
In parts of Woking, south of London, residents will have a Covid-19 test posted through their mailbox and asked to use it. The tests will be collected by health teams later the same day.
“This is a precautionary measure – the more cases of the variant we find, the better chance we have at stopping it from spreading further,” said Ruth Hutchinson, the director of public health for Surrey.
“By playing your part and taking the test, you’ll be helping to keep your community and your loved ones safe.”
Last week, Health Secretary Matt Hancock said all the cases of the new strain were connected to travel from South Africa.
Former UK chief scientific adviser Prof Mark Walport said the success of surge testing relied upon uptake.
He said it was now relatively easy for labs to work out whether someone had been infected with the South African variant.
“It’s important people co-operate. It’s in their interests to do so. The UK is in a unique position to do this screening,” he said.
“The challenge is to hold back the variants as long as we possibly can.”
UK health officials previously expressed concern that Covid-19 vaccines may be ineffective against the new strain.
A South African study found that the strain could be resistant to existing immunity, while two British scientists suggested the variant may be 30 per cent deadlier than the original virus.
Boris Johnson ‘confident’ over vaccines
Last week, Prime Minister Boris Johnson banned travel to the UK from more than 30 “red list” countries where there is concern new strains may be spreading.
Britain's vaccination campaign is making progress and hit the "crucial milestone" of offering a shot to every care home resident in England.
On Saturday, a record 598,389 shots were given to patients across the UK, with nearly nine million people receiving the first dose of a vaccine.
Mr Johnson said he was confident Covid-19 vaccines could be adapted to new variants of the virus if necessary.
“We are confident that all the vaccines that we are using provide a high degree of immunity and protection against all variants,” he said on Monday.
“The fact is we are going to be living with Covid for a while to come, in one way or another. I don’t think it will be as bad as the last 12 months – or anything like – but it’s very, very important that our vaccines continue to develop and to adapt, and they will.”
Dr Mike Tildesley, an infectious disease modeller from the University of Warwick, said the lifting of lockdown restrictions would depend on how effective vaccines are at preventing transmission of the virus.
"If we think optimistically, if we follow the trajectory of the rollout of the vaccine … then hopefully we can be easing out of these controls that we've got in place sometime in March," he told BBC Radio 4's Today programme.
“But I think even with that optimistic situation, it needs to be done relatively gradually. The danger is, of course, as we do start to unwind controls then we offset the gains that we get from vaccination, so we need to be very careful.
“But if the vaccine rollout continues at high levels, and we do find that actually these vaccines are very good at blocking transmission as well as preventing severe infection, then we’re in a good position.
“Hopefully by the summer we can get back to something pretty close to what we have seen before the pandemic was normal.”
Dr Tildesley said it was vital for the UK “to have these vaccines being pretty good at blocking transmission”.
Meanwhile, South Africa received its first Covid-19 vaccines produced by the Serum Institute of India.
About one million AstraZeneca doses will be used to inoculate South Africa’s frontline health workers in mid-February, after the vaccines are tested and approved by the country’s medicine regulator.
The government there intends to inoculate 40 million people, representing 67 per cent of the population, by the end of the year.
Last week, South African president Cyril Ramaphosa accused rich countries of hoarding Covid-19 vaccines at the expense of the world's poorest people.
