Surge testing is being rolled out in parts of south London and Cumbria amid a rise in cases of the more transmissible Delta variant, first identified in India.
Residents in Lambeth are strongly encouraged to take a Covid-19 PCR test, whether or not they have symptoms.
The enhanced testing will begin on Saturday in Clapham, Brixton, Stockwell, West Norwood and Vauxhall.
So far, all the confirmed cases have been instructed to self-isolate and their contacts have been identified, the Department of Health and Social Care (DHSC) said.
Surge testing will also begin in Cumbria where everyone aged 12 to 30 who lives, works or studies in the area is strongly encouraged to take a PCR test, whether they are showing symptoms or not.
Those of school age will be offered testing at school.
The DHSC said people in these areas should continue to make use of free twice-weekly lateral flow tests alongside the PCR test as part of surge testing.
Meanwhile, giant jab clinics are open in London this weekend, with the Olympic Stadium, Tottenham Hotspur FC, Chelsea FC and Charlton Athletic FC among those that will host large scale pop-up clinics for residents, with smaller events taking place in local community venues.
It comes as officials are reportedly considering scrapping the 10 days of self-isolation for people who have been double vaccinated and come into contact with someone infected with Covid-19.
The Times reported that Health Secretary Matt Hancock is said to be keen to replace quarantine with daily testing, but that the policy will be approved only after Professor Chris Whitty, the chief medical officer for England, is satisfied with the results of a study of 40,000 people.
