Britons in Portugal say they are anxiously waiting a decision by the UK government on whether to reintroduce quarantine for arrivals.
Cases in Portugal have risen in the past week beyond the threshold at which ministers consider imposing 14-day mandatory self isolation.
Health Secretary Matt Hancock said a decision based on data – including case numbers – would be taken later.
Meanwhile, Scotland has reintroduced isolation for arrivals from Greece.
In Wales, arrivals from the Greek island of Zakynthos have been told to quarantine for 14 days following a number of cases linked to the popular tourist destination.
Changes to the rules for arrivals from Greece coming to England will also be considered by ministers on Thursday.
Laura McDonald, 26, who is with her fiancé Jake Robson, 27, in the Algarve, told BBC News it would cost them £450 to get home early – despite originally paying just £230 in total for their return flights.
“It’s just not feasible. What we would pay is half of what we lose in wages each,” she said.
Ms McDonald said the resort of Albufeira already seems quieter following speculation earlier this week about the possibility of having to quarantine on arrival back in the UK.
“We were on the beach yesterday and there were a lot of British people there talking about it and reading the news,” she said.
“Whilst we are having a great time it is at the back of our mind and we are anxiously awaiting the next announcement,” she added.
The couple now face the prospect of a fortnight without pay, or the loss of holiday allowance, if they have to self-isolate when they get back to Newcastle on Monday.
Gary Hiles, 34, and his family face the prospect of a second holiday being cancelled this year due to coronavirus restrictions after booking a trip to Portugal last month.
“We have an autistic son, who we’ve been trying to prepare for the holiday, but now we don’t know if we are going or staying,” Mr Hiles said.
“He keeps getting excited about going on a plane. He doesn’t understand why our holiday may be cancelled.”
Some holidaymakers have told the BBC they have paid as much as £1,000 for flights amid a scramble to get home from Portugal this week.
Portugal was taken off the UK government’s list of countries requiring quarantine for arrivals on 22 August following “intense bilateral work” between the two countries.
In recent weeks, the Department for Transport (DfT) has announced changes to its quarantine exemption list on Thursday nights, with the new rules being imposed on Saturdays at 04:00.
In Portugal, the seven-day infection rate has increased from 15.3 to 23 per 100,000 people.
A seven-day rate of 20 is the threshold above which the UK government considers triggering quarantine conditions.
Every year, more than two million Britons visit Portugal, making up the largest number of overseas tourists to the country.
Most head to the Algarve in the south, drawn by sunny Atlantic beaches, picturesque fishing villages and golf courses.
During May and June, the Portuguese government reopened its restaurants, coffee shops, museums and beaches. Hotels have mainly reopened, but nightclubs remain closed.
In Greece, overall, there were 13.8 coronavirus cases per 100,000 people in the seven days to 2 September, down from 14.9 a week earlier.
However, holiday giant Tui temporarily ended package tours to the Zakynthos resort of Laganas following a number of cases linked to the town.
Greece has insisted it is doing “everything in our power” to keep UK holidaymakers safe.
Last week, the UK government reimposed a mandatory quarantine for arrivals from Switzerland, Jamaica and Czech Republic – but removed the requirement for arrivals from Cuba.