The South African variant of the coronavirus was detected in a major London borough on Wednesday, prompting fears of a surge in infections.
"The South African variant of Covid-19 has been found in Barnet ... we will start testing people for this variant in specific postcode areas ...," a statement by the local authority said.
It comes after 44 confirmed and 30 probable B.1.351 cases were identified in Wandsworth and Lambeth boroughs.
The development follows a warning from a leading scientist that the rapid spread of variants could force the government to reimpose lockdown measures and reverse the progress made in the last three months.
"A lot of we scientists are very concerned about what’s happening at the moment. I think we’re all just hoping that the staged reduction in lockdown is going to be ok. It is being done reasonably cautiously but I think this is not good news," immunologist Peter Openshaw told BBC2's Newsnight.
"If we get rapid spread of the South African or other more resistant variants, it may well be that we are going to have to put the reductions of lockdown into reverse."
With 2,491 people testing positive in the last 24 hours, the overall caseload in the UK has risen to over 4.38 million, including 127,407 related deaths.
Over 32 million people have been given their first dose of a COVID-19 vaccine, while 8.1 million have been fully vaccinated.
The latest R range for the UK stands at 0.8-1.0, with the current growth rate at -4% to 0% per day.
Shops and hairdressers reopened in England on Monday after months of lockdown.