Surge testing is being rolled out in an Essex town where a case of the South African variant has been found.
The single case was detected in an area of Brentwood. People living and working in the CM13 postcode are now being urged to take the test when offered it.
Essex County Council said all households would get a letter to say where and when they could get a test.
Dr Mike Gogarty, director of public health in Essex, said it was "vital" for anyone over 16 to be tested.
He said officials wanted to "break the chain of transmission as soon as possible" and asked anyone who had tested positive for Covid-19 to self-isolate immediately.
Dr Gogarty said: "Do not be alarmed but, even if you are feeling well, I would urge everyone to play their part to keep their community safe.
"If you receive a letter, please book a Covid-19 test as soon as possible."
The case has not been linked to international travel.
Anyone with symptoms of Covid-19 has been advised to book a test in the usual way, with four mobile sites open in the area seven days a week.
These sites are based at Shenfield High School, Thorndon Country Park, Hutton Adult Community Learning car park and The Brentwood Centre.
Testing kits can also be collected from the Hutton Adult Community Learning car park.
Elsewhere in the region, Norfolk County Council said it has had a "fantastic response" after deploying surge testing in the IP22 postcode for Diss and Roydon.
Five thousand people came forward on Friday's first day of testing after a "very small number" of South African variant cases were discovered.
"Precautionary" testing will go on for a further 12 days and the council hoped a total of 10,000 people would be tested.
Testing will be carried out at mobile units or with a kit at home.
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