Wales’ first town-only lockdown will come into force later.
Llanelli in Carmarthenshire will have restrictions imposed from 18:00 BST, making it the first town hit with restrictions without the rest of the county it belongs to.
Wales’ two biggest cities - Cardiff and Swansea - will follow suit 24 hours later following Covid-19 spikes.
People in 13 ward areas in Llanelli cannot leave town, or mix indoors with anyone outside their own household.
The town has seen 85 coronavirus cases over the past week - compared to 24 across the rest of Carmarthenshire.
“It is worrying to see how sharply the number of positive cases has risen in the Llanelli area and action has had to be taken to help stop the spread and break the chain of infections concentrated in this area to prevent a whole county lockdown,” said Carmarthenshire council leader Emlyn Dole.
“We must all do the right thing, follow the advice and protect each other.
“In parts of Llanelli, we’re asking people and businesses to make even greater sacrifices - we fully appreciate the impact this will have, but there is no other way. We must stop the spread.”
The rate of infection across the town has leapt to 152 cases per 100,000 of the population - it is just 18 per 100,000 for the rest of the county.
It places Llanelli in the top three weekly infections rates across Wales, alongside Blaenau Gwent on 202 per 100,000 and Merthyr Tydfil at 169 per 100,000.
Neighbouring Swansea hit a seven-day rate of 56 per 100,000 on Friday, while Cardiff reached 38 cases per 100,000.
The two areas will face the same additional coronavirus restrictions as Llanelli from 18:00 on Sunday.
Chairwoman of Hywel Dda University Health Board, Maria Battle, said: “Our local community has given us such tremendous support during the past few months.
“To protect the health of our people, including the most vulnerable, and to ensure our NHS resources are available to provide people with the care they need, we need the help of our Llanelli population and wider community now more than ever before.”
Extra testing capacity is being introduced, with appointments at Parc y Scarlets, Ty’r Nant at Trostre, and the Carmarthen showground.
Health officials said there should be “no reason for Llanelli residents to travel excessive distances for a test”.