Travellers from Portugal will have to isolate when arriving in Scotland and Wales but not England, sparking confusion.
The nations made different decisions at the weekly point when their quarantine lists are normally updated.
There was speculation about what would happen to Portugal given COVID-19 cases there rose to 23 per 100,000 people in the seven days up to Wednesday – up from 15.3 a week earlier.
The UK government generally says countries where the figure is higher than 20 per 100,000 are at risk of being added to the list.
In Scotland, anyone arriving from Portugal or French Polynesia must quarantine after 4am on Saturday. Greece was added to its quarantine list earlier this week.
And in Wales, travellers will have to isolate for 14 days after 4am on Friday if coming back from Portugal – excluding the Azores and Madeira islands – as well as Gibraltar, French Polynesia and the Greek islands of Mykonos, Zakynthos, Lesvos, Paros, Antiparos and Crete.
Transport Secretary Grant Shapps, who is responsible for England only, tweeted the quarantine list is kept “under constant review” and he would not “hesitate to remove countries if needed”.
But he added “there are no English additions or removals today”, though reminded tourists that destinations exempt from quarantine “can and do change at very short notice”.
Northern Ireland said there will be “no changes to the current travel quarantine rules”.
Holidaymakers in Portugal were scrambling to get home this week.
British Airways flight prices jumped to more than £500 for Thursday, and EasyJet sold out all its flights from Faro to airports in Britain for Wednesday, Thursday and Friday.
It is understood ministers from all four UK nations met this evening to discuss the situation.
Kelly Jones changed her family’s flights back from the Algarve from Saturday to Friday to ensure the children would not miss out on two weeks of school.
The 45-year-old said she paid Jet2 £900 on Monday to bring the trip home to Birmingham forward a day in anticipation of quarantine measures being reintroduced and hit out at the “absolutely disgusting” situation.
“The government just change the goalposts left, right and centre at the moment,” she said. “It’s embarrassing.
“They don’t make things clear enough. They say they need the rate under 20 and it’s gone above that this week, so why wouldn’t you think it would go on the quarantine list tonight?
“You can guarantee if I hadn’t have booked those flights on Monday, that air bridge would have been removed tonight.”
Derek, who was visiting Almancil in Portugal, said 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”.
Rory Boland, editor of Which? Travel, criticised ministers for offering “no clarity around how these decisions are made” and “ignoring the growing evidence suggesting this system is not working”.
“If the government is serious about letting international travel resume while prioritising public health, a major reassessment of its approach is needed,” he added.
And Paul Charles, chief executive of travel consultancy The PC Agency, said: “The quarantine policy is in tatters and dividing the UK.”
Portugal was originally left off the list of so-called “travel corridors” where people could arrive from without needing to quarantine, when the policy was brought in in July.
It was only added to the safe list a few weeks ago – on 22 August.
A series of European destinations have already been impacted by sudden quarantine announcements this summer, such as Spain, France and Croatia.
Ministers say they need to act quickly so the measures are effective.