Surge testing is being introduced to parts of west London after cases of the Indian coronavirus variant were detected.
Local authorities will “shortly confirm” the areas where additional testing will be offered within those boroughs, the Department of Health and Social Care (DHSC) said.
Those who tested positive for the variant have been told to self-isolate and their contacts are being identified.
People who have symptoms can book free tests online or by phone, while those without symptoms are advised to visit their local council’s website for more information.
Along with increased testing in the boroughs, so-called “enhanced contact tracing” – whereby tracers look back over an extended period of time to determine the route of transmission – will be used for those who test positive with a variant of concern.
Cases of the Indian variant of coronavirus in the UK have risen by more than 2,000 in the space of a week, according to figures from Public Health England (PHE).
The latest weekly data showed there were 3,424 cases of the B1617.2 mutation – an increase of 2,111 on the previous week.
While most cases were concentrated in the North West – particularly Bolton – and London, PHE said it was seeing “clusters of cases” across the country.
Dr Meera Chand, the Covid-19 incident director at PHE, said it was essential people in the worst-affected areas who had yet to receive their second dose of the vaccine came forward as soon as it was offered.
“This is vitally important in the light of our current assessment that (B1617.2) has grown rapidly in England and may be highly transmissible,” she said.
Figures released on Friday revealed that in London cases of the Indian variant have increased from 400 to 720.
On Thursday, Public Health England put Hounslow in sixth place on its national Covid “watch list”.
The Mayor of London, Sadiq Khan, said: “Due to the spread of cases of the Covid-19 variant of concern first discovered in India, enhanced testing will take place in targeted settings in four more boroughs in the capital.
“Hillingdon, Harrow, Brent and Ealing will join Hounslow in introducing targeted surge testing to identify this variant and stop it spreading further.
“I’ve met with NHS London, Public Health England and local health officials, and together we will continue to take the necessary action to protect Londoners.
“I urge everyone who lives, studies or works in these five boroughs to visit the website of their local authority for more details and to take a test as soon as they are offered it.
“It’s vital that everyone also accepts the vaccine as soon as possible and I continue to urge the Government to increase the availability of the vaccine to younger age groups who live where this variant is spreading.
“We’ve worked incredibly hard to reduce the number of cases in London and it’s important we all continue to do what we can to reduce the spread of the virus.”
About a quarter of the first 400 cases in the capital were linked to international travel, with the majority of the remainder being family members or close contacts.
In Hounslow, checks are being done on “quarantine hotels” to establish if people harbouring the virus have passed it to hotel staff and then on into the community. Concerns about these hotels were raised in Parliament earlier this week by Brentford and Isleworth MP Ruth Cadbury.
Vaccination staff in the five boroughs are said to be taking a pro-active approach to offering jabs to younger Londoners who may not be entitled to receive a vaccine on the basis of their age.
This is to limit the spread of infection in multi-generational households that are common across Asian communities in west London, and to avoid turning away people who may otherwise be hard to reach.