Experts are seeking to urgently test 80,000 people in England for Covid-19 after it emerged that the South African strain of the virus may have spread in some regions.
Health officials said 11 people had been identified over the last five or six days who have tested positive for the variant, but who have no links to travel.
This suggests there may pockets of spread in local communities of the new variant, with the possibility of further cases.
Experts from Public Health England (PHE), who have been sequencing around 5% to 10% of all positive cases looking for variants, are now hoping to break any chains of transmission. All but two areas involved single cases of the variant.
Mobile testing units and some home testing kits are being sent into eight postcode areas of London, the West Midlands, East of England, South East and the North West.
The areas are: Hanwell, west London; Tottenham, north London; Mitcham, south London; Walsall in the West Midlands; Broxbourne, Hertfordshire; Maidstone, Kent; Woking, Surrey; and Southport, Merseyside.
People will be urged to agree to testing, whether they have symptoms or not.
The South African variant is thought to be as transmissible as the variant that was first identified in Kent but there is no evidence as yet that it causes more severe disease.
It is not yet known whether the Oxford University/AstraZeneca vaccine will be effective against the variant, although other vaccines have shown promising results.
Experts advising the Government said they did not think the current vaccines would need to be tweaked to deal with any spread of the South African variant.
To date, 105 cases of the South African variant have been identified in the UK since December 22 but all of those had links to travel.
Experts believe the 11 new cases may also have second or third generation links to travel but detailed investigations have not identified any such links.
Health and Social Care Secretary Matt Hancock said: "It is vital that we do all we can to stop transmission of this variant and I strongly urge everyone in these areas to get tested, whether you have symptoms or not.
"The best way to stop the spread of the virus – including new variants – is to stay at home and follow the restrictions in place. Until more people are vaccinated this is the only way we will control the spread of the virus.
"The UK is a global leader in Covid-19 genomics and, because of this, we have been able to identify new strains of the virus and take decisive action.
"We continue to closely monitor new variants, here and around the world, and in addition to our already extensive testing service, we are making surge-testing capacity available to affected areas."
Dr Susan Hopkins, strategic response director at PHE, said: "As part of our proactive sequencing work, we know that the new variant of Covid-19 first detected in South Africa has been identified in a number of areas across England.
"A small proportion of these cases have no link to international travel suggesting that there are some cases in the community.
"In response to this, we are ramping up testing in targeted areas, so we can gather more information and effectively monitor any further community transmission."
Ealing Council said it was asking residents living and working in parts of Hanwell and West Ealing to get a Covid-19 test after one local resident there tested positive for the South African strain.
Ealing Council is asking residents living and working in parts of Hanwell and West Ealing to get a COVID-19 test. This applies whether they have symptoms or not, after a local resident tested positive for the South African strain of the virus. https://t.co/eJRluKICjEpic.twitter.com/N9ujjpcHKE
— EalingCouncil (@EalingCouncil) February 1, 2021
In Kent, one case has also been identified with no previous links, while Surrey County Council said testing would occur in the Goldsworth Park and St Johns areas of Woking.
Birmingham City Council's director of public health, Dr Justin Varney, said while there had been one case in Walsall "there will be others in the West Midlands I'm sure."
It comes as Boris Johnson played down fears about vaccines being ineffective against different variants.
During a visit to the Al Hikmah vaccination centre in Batley, West Yorkshire, the Prime Minister told reporters: "We are confident that all the vaccines that we are using provide a high degree of immunity and protection against all variants."
He said the vaccines could be adapted to deal with new variants if necessary.
"The fact is we are going to be living with Covid for a while to come in one way or another," he added.
"I don't think it will be as bad as the last 12 months – or anything like – of course, but it's very, very important that our vaccines continue to develop and to adapt, and they will."
Mr Johnson said there were signs the lockdown measures were working but it was too early to "take your foot off the throat of the beast" by easing restrictions.
"We are starting to see some signs of a flattening and maybe even a falling off of infection rates and hospitalisations," he said.
"But don't forget that they are still at a very high level by comparison with most points in the last 12 months, a really very high level.
"So the risk is if you take your foot off the throat of the beast, as it were, and you allow things to get out of control again then you could, alas, see the disease spreading again fast before we have got enough vaccines into people's arms. That's the risk."
Mr Johnson said he had not taken a decision on whether there would be a return to the tier system following the lockdown.
"It may be that a national approach, going down the tiers in a national way, might be better this time round, given that the disease is behaving much more nationally," he said.
"If you look at the way the new (UK) variant has taken off across the country, it's a pretty national phenomenon.
"The charts I see, we're all sort of moving pretty much in the same sort of way, I mean there are a few discrepancies, a few differences, so it may be that we will go for a national approach but there may be an advantage still in some regional differentiation as well. I'm keeping an open mind on that."
Mr Johnson was asked if borders could fully reopen once the adult population is vaccinated and if the vaccines were shown to be good enough to deal with mutant strains.
He said: "We will be looking very closely at the impact of the vaccines on the patients, on the population, how effectively they're getting the deaths down, how effectively they're reducing serious illness and will be taking a series of judgments about when and how to relax.
"What I don't think people want to see is a speedy relaxation that leads to an upsurge in the virus and I think everybody understands the need for tough border controls whilst there is a risk of new variants coming in. We want to take no risks with our borders."