The largest surge testing operation so far is taking place in areas of south London after more than 70 confirmed or probable cases of the South African variant of COVID-19 were detected.
The government says dozens of people are isolating or have completed their isolation after contracting the variant, B.1.351, and the cluster of infections is believed to be "significant".
NHS Test and Trace is providing extra testing and genomic sequencing, mostly in the boroughs of Wandsworth and Lambeth.
There have been 44 confirmed cases and 30 probable cases predominantly found there.
People aged 11 and over who live, work or travel through those areas are being urged to take a COVID-19 PCR test, on top of using twice-weekly rapid testing.
It comes as scientists in Israel said the variant is able to "some extent break through" the Pfizer/BioNTech vaccine’s protection.
The Department of Health and Social Care said it is the "largest surge testing operation to date" in south London aimed at suppressing any possible new cases of the variant.
Extra testing sites will be made available and positive results from PCR testing will be sent for genomic sequencing at specialist labs.
It said all those who have tested positive for the variant, with the first case in the area being found in early March, are isolating or have completed their isolation, and their contacts have been traced and asked to isolate.
Facilities offering asymptomatic PCR testing have been deployed at Wandsworth Town Hall, Tooting Leisure Centre and the University of Roehampton, and further testing sites will be opened during the week.
Vauxhall MP Florence Eshalomi tweeted: "It is important for everyone over 11 across Lambeth to get a COVID PCR test ASAP – we have to keep safe and help stop the spread of the virus."
Dr Susan Hopkins, chief medical adviser for NHS Test and Trace, said the cluster of cases is "significant".
"It’s really important people in the local area play their part in stopping any further spread within the local community," she said.
"PCR testing is now available for all and I would strongly encourage everyone, whether they live, work or travel through the boroughs, to get tested even if they don’t have any symptoms of coronavirus.
"Around one in three people with COVID-19 don’t show any symptoms.
"By taking part you can protect yourselves and your loved ones and help us identify any possible new cases that would otherwise be missed, preventing further transmission and saving lives."
The Israeli study compared nearly 400 people who had tested positive for COVID, 14 days or more after they received one or two doses of the Pfizer vaccine, against the same number of unvaccinated patients with the disease.
The South African variant was found to make up about 1% of all the COVID-19 cases across all the people studied, according to the Tel Aviv University study.
But among patients who had received two doses of the vaccine, the variant’s prevalence rate was eight times higher than among the unvaccinated – 5.4% versus 0.7%.
This suggests the vaccine is less effective against the South African variant, compared with the original coronavirus and a variant first identified in Britain, the researchers said.
Tel Aviv University’s Adi Stern said: "We found a disproportionately higher rate of the South African variant among people vaccinated with a second dose, compared to the unvaccinated group.
"This means that the South African variant is able, to some extent, to break through the vaccine’s protection."
The study has not yet been peer reviewed.