Wales’ two biggest cities are preparing for lockdown, which begins later.
The changed status of Swansea and Cardiff will take the number of Welsh local authority areas under heightened Covid restrictions to eight.
It follows the first localised lockdown in Wales, in the town of Llanelli in Carmarthenshire, coming into force on Saturday evening.
It means 1.5 million people - about half of Wales’ population - will be under lockdown by 18:00 BST on Sunday.
The restrictions will be the same as those affecting people living in Merthyr Tydfil, Bridgend, Blaenau Gwent, Newport, Rhondda Cynon Taf and Caerphilly, which are already in lockdown.
Swansea hit a seven-day rate of 56 new cases of coronavirus per 100,000 on Friday, while Cardiff reached 38 cases per 100,000.
But the number in Llanelli was 152 cases per 100,000 - which is why the Welsh Government decided to bring in restrictions there a day earlier.
The whole of Carmarthenshire was not put into lockdown because the rate in the rest of the local authority area was 18.
Llanelli town is in the top three places with the highest weekly infections rates, alongside Blaenau Gwent on 202 per 100,000 and Merthyr Tydfil at 169 per 100,000.
Under the rules, nobody is able to enter or leave the eight counties and one town without a “reasonable excuse”.
People are allowed to travel outside the area for a limited number of reasons.
These include going to work if they are not able to work from home, to go to school, give care and buy food or medical supplies.