Travellers from mainland Portugal to England will have to quarantine from 04:00 BST on Saturday, just weeks after the country was put on the safe list.
Meanwhile, Sweden has been added to the list of countries exempt from quarantine for England and Wales.
Portugal, one of the UK’s most popular holiday destinations, was given an exemption from UK quarantine rules as recently as 22 August.
But since then the infection level has been rising. Last week the seven-day average exceeded 20 cases per 100,000 people, which the UK generally uses as a threshold for adding countries to the quarantine list.
Transport Secretary Grant Shapps said that because of « enhanced data » the government was able to assess the infection rates of islands separate to their mainland countries.
He also stressed that travellers must fill out the Passenger Locator Form on arrival to the UK, a day after the prime minister said enforcement of this part of the quarantine rules would be stepped up.
« It is a criminal offence not to complete the form and spot checks will be taking place, » Mr Shapps said.
People travelling to England and Wales from Hungary and Reunion will also be required to self-isolate, while England is additionally requiring arrivals from French Polynesia to quarantine.
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