Portugal is increasingly likely to be removed from the Government’s quarantine exemption list after its coronavirus case rate worsened.
There were 23.0 Covid-19 cases per 100,000 people in the country in the seven days to Wednesday, up from 15.3 a week earlier.
Pressure is also mounting on England to consider imposing quarantine rules on travellers arriving from Greece, amid reports of people returning with coronavirus.
The Scottish Government is requiring travellers from the country to self-isolate for 14 days from Thursday, while Wales has imposed the measure on arrivals from the island of Zante.
Health Secretary Matt Hancock said a decision on whether to impose restrictions on the countries would be announced on Friday.
In Greece, the seven-day coronavirus case rate is 13.8. It is 15.3 in Ireland and 14.7 in the UK.
Figures have been calculated by the PA news agency based on data collected by the European Centre for Disease Prevention and Control.
A seven-day rate of 20 is the threshold above which the UK Government has considered triggering quarantine conditions.
One family due to fly to Portugal this weekend now faces the prospect of a second holiday cancelled due to coronavirus restrictions.
Gary Hiles, 34, from Sunderland and his family originally booked to go to Alcudia in Majorca but were forced to cancel after Spain was removed from the list of exempt travel destinations.
They then rebooked to Portugal because the country had recently been added to the list.
Mr Hiles told PA: “We (initially) booked in July 2019, it was supposed to be our first family holiday abroad for our children.
“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. He keeps getting excited about going on a plane. He doesn’t understand why our holiday may be cancelled.”
Holidaymakers booking flights home this week to avoid a potential quarantine requirement have been paying hundreds of pounds each due to high demand.
British Airways was selling air fares from Faro – which serves Portugal’s Algarve – to Heathrow for as much as £554.
In Buckinghamshire, two groups of people have tested positive for coronavirus after returning from abroad – including a group of teenagers from the Marlow area who are reported to have been on a trip to Zante.
Buckinghamshire Council said some of them were pupils at Sir William Borlase’s Grammar School and all the affected households were self-isolating.
Tour operator Tui also said it was aware of a “small number” of positive Covid-19 test results following a flight from Heraklion in Crete to London Stansted on August 27.
One passenger on the flight, who had been contacted by NHS Test and Trace and asked to self-isolate, complained that “hardly anyone” on the flight had been wearing a mask.
The woman, who wished to remain anonymous, told PA: “Our flight was hideous, hardly anyone wore a mask and the crew didn’t do anything about it.
“We raised the issue twice with the cabin crew who still didn’t enforce it.
“My family is OK but we are having to self-isolate.
“I’m so sad for my son because he has to miss his first week back at school.”
A Tui spokeswoman added: “We can confirm that no guests tested positive for Covid-19 in our hotels and no symptoms were displayed before or during the flight home by any of the customers.”