Travellers from Portugal arriving in Wales and Scotland must now spend a fortnight in isolation – but there are still political divisions over the decision.
The rules came into force from 4am on Saturday, but do not apply to England and Northern Ireland.
Travellers from Greece were told to isolate from Thursday in Scotland.
Portugal had only been removed from the UK quarantine list two weeks ago but there had been suggestions the Department for Transport would opt to reimpose it after coronavirus cases rose above 20 cases per 100,000 people – the threshold used for past travel decisions.
Fresh restrictions have already been reintroduced for the popular tourist destination by the Welsh government but England and Northern Ireland opted not to collapse the so-called travel corridor with Portugal.
Scotland on Thursday reintroduced quarantine for those returning from Greece and while Wales did the same for six Greek islands, including Crete and Zante, but England and Northern Ireland resisted.
Despite the differences, Prime Minister Boris Johnson insisted that “overwhelmingly the UK is proceeding as one”.
But earlier in the day, Transport Secretary Grant Shapps said there was now “confusion” over differing quarantine rules across the union and accused Scotland of having “jumped the gun” on imposing restrictions on Greece.
A Scottish government spokesman hit back saying there had been a “worrying number of people in Scotland who tested positive for COVID-19 and who had returned from Greece”.
:: Subscribe to the Daily podcast on Apple Podcasts, Google Podcasts, Spotify, Spreaker
He added: “In the case of Portugal, it was unfortunate that the UK government announced their decision yesterday before ministers from England, Scotland and Northern Ireland met and before considering the latest Joint Biosecurity Centre (JBC) data.
“This indicated a significant rise in both the prevalence of the virus in Portugal and in test positivity.”
Welsh health minister Vaughan Gething said Wales was acting “in line with” the risk assessments from the JBC in imposing a 14-day quarantine on arrivals from the six Greek islands and Portugal.
Holidaymakers in Portugal reacted with confusion when the rules were announced, with many already having booked earlier flights home.
Derek, who was visiting Almancil in Portugal, told Sky News that his son and wife did not join him because they feared quarantine being imposed and his brother had rushed home to go back to work and help his son head to university.
He claimed tourists were being left “confused” and that “it’s very unfair because people seem to be put on short notice, not only is it very expensive but they don’t know if they’re coming or going”.