Surge testing has begun in third and fourth areas of London after more cases of the “South Africa” Covid variant were detected.

People living in part of the SE16 postcode area of Southwark will be asked to take a PCR test whether they are showing symptoms or not, while those in the N3 area of Barnet, north London, are being offered the same.

The government said the Southwark case, identified on Tuesday, was linked to a cluster identified in neighbouring Lambeth and Wandsworth on Monday. Barnet Council, which announced its own case, said it was unrelated to the cluster detected in south London. 

The largest surge testing operation to date was launched on Monday after 44 confirmed cases of the variant were found, with another further 30 probable cases.

Anyone who lives or works in either borough was asked to get a test, with long queues forming on Tuesday.

The South Africa strain is classed as a “variant of concern” – the most serious classification issued by Public Health England – because there are fears it may be less susceptible to vaccines and could spread more easily.

PHE’s regional director in London Dr Kevin Fenton said: “The cluster of cases in parts of south London – predominantly the Lambeth and Wandsworth areas but also in Southwark – of the variant first identified in South Africa is significant. It’s really important people in the local area play their part in stopping any further spread within the local community.”

Affected households in Southwark will either get a leaflet asking them to get a test from a mobile unit, or be asked to take a home kit by a door-to-door team.

Ministers are already under pressure to tighten quarantine laws due to the outbreak in Lambeth and Wandsworth.

Downing Street insisted the outbreak was being taken “very seriously” and “strong measures” had been put in place to prevent the spread of the variant.

But Labour said the development was “deeply concerning” because of concerns that vaccines could be less effective against variants.

Shadow home secretary Nick Thomas-Symonds has written to Priti Patel to call for a more “comprehensive” hotel quarantine system.

The BBC reported that the outbreak appears to have been triggered by an individual who travelled from Africa in February.

According to documents seen by the broadcaster, the country involved was not on the red list for mandatory hotel quarantine at that time, but is now.

Boris Johnson said on Tuesday that he could not “see any reason” to deviate from the road map, which would see further lockdown restrictions lifted on May 17 in England.

For people who test positive, a “comprehensive” self-isolation support service is available which includes payments of up to £500 for residents on low incomes, though throughout the pandemic people have struggled to actually get the cash.

The Department of Health and Social Care said positive results from PCR testing would be sent for genomic sequencing at specialist labs – the only way to detect which variant people have after they test positive.

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.

The confirmed case in Southwark is self-isolating and their contacts have been identified.

The government’s public tracker for the most potentially serious categories of variants – “variants under investigation” and “variants of concern” – reveals that the South Africa variant had been the second most widespread in the UK even before the recent outbreak, with 544 confirmed cases up to April 7.

A variant discovered in the UK and Nigeria dating back to December is the third most common, with 334 cases in Britain.

The so-called “Kent variant”, by contrast, has now been detected nearly 200,000 times. Because only a handful of positive Covid tests undergo genomic sequencing the true number is likely to be much higher.

The Kent variant is thought to be one of Britain’s dominant strains, having been blamed for much of the second wave that left Britain in lockdown from December. It is not, however, believed to be any less susceptible to vaccines.