Surge testing has been introduced to parts of in London following the detection of the South African coronavirus variant.
Genomic sequencing in the areas will also be ramped up as part of efforts to control and suppress the spread of the South African variant, which has been shown to evade parts of the immune response triggered by the Covid-19 vaccines.
The Department for Health and Social Care, which made the announcement on Monday, did not say how many cases linked to the South African variant had been identified.
Positive cases that are picked up in Harrow and Southwark will be assessed to determine which form of the coronavirus was responsible for the infection.
For those individuals who test positive for any of the known variants of concern, enhanced contact tracing will be implemented, the DHSC added.
This involves contact tracers looking back over an extended period in order to determine the route of transmission.
People with symptoms in SE5, HA2 and HA3 should book a test in the usual way, the DHSC said, and those without symptoms are being asked to visit their local authority website for more information.
According to data from Public Health England, a total of 318 infections caused by the South African variant have been picked up across the country to date.
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