Ministers are under pressure to tighten quarantine laws after dozens of cases of the South African coronavirus variant were found in two south London boroughs.
Extra testing facilities were set up in Wandsworth and Lambeth after a cluster of cases were detected, with the first case in the area being detected in March.
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 “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.
Some 44 confirmed cases of the variant have been found in Lambeth and Wandsworth, with a further 30 probable cases identified, the Department of Health and Social Care (DHSC) said.
Lambeth Council said eight additional testing sites have opened across the borough from Monday and facilities are also available in Wandsworth.
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.
Director of public health for Lambeth, Ruth Hutt, said: “It is vital that all Lambeth residents take an asymptomatic PCR test so that we can identify all cases in order to stop the spread of this variant of concern.
“Alongside taking a PCR test, we are asking everyone who lives and works in the borough to continue following the guidance in place to stop the spread of Covid-19.
“That includes limiting your contacts with other people, washing your hands regularly and thoroughly, keeping your distance and covering your face.
“If you test positive for Covid-19 via a PCR test or rapid lateral flow test it is crucial that you self-isolate so you don’t pass on the virus, and support is available to help you do that.”
Labour MP Fleur Anderson for Putney, Roehampton & Southfields said she has raised the need for more testing sites and longer opening hours with Wandsworth Council.
She said: “I urge everyone to get a PCR test and let their friends and neighbours know as well. I have raised the need for more testing sites with Wandsworth Council, and been assured that these will be made available very soon, as well as access to tests for people who are house-bound.
“Parents of children aged over 11 in schools outside Wandsworth or Lambeth need to book their children in for a PCR test or pick one up as well as these are different to the lateral flow tests they have at school.
“The more people [that] get a PCR test the more we can stop the spread of this variant, save lives and keep opening up and getting back to normal.”
The DHSC confirmed that all those who have tested positive for the variant, are isolating or have completed their isolation, and their contacts have been traced and asked to isolate.