Airlines are laying on extra capacity to bring people back to the UK from Portugal before the country is removed from the travel green list on Tuesday.
British Airways is increasing its flights, while Tui and EasyJet are upgrading the size of their planes.
The move to the amber list means UK tourists should not visit the country and returnees must isolate for 10 days.
But travel firm Tui says 50% of passengers due to travel there in June are still planning to go.
Any travellers scrambling for return flights before the new rules come in at 04:00 BST on 8 June are facing steep prices for tickets – with a single BA ticket from Faro to London City going for more than £540.
Health Secretary Matt Hancock said the decision on Portugal, which includes the islands of Madeira and the Azores, had to be made because of a rise in cases of the Delta variant, first detected in India.
“It was not a decision I wanted to take,” he said. “But we’re absolutely determined to keep this country safe, especially from novel variants coming from overseas.”
Speaking from a G7 health ministers’ meeting in Oxford, Mr Hancock added that “tough” rules on international travel were also necessary to protect the success of the UK’s vaccine rollout.
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However, Portugal’s minister of foreign affairs, Augusto Santos Silva, told the BBC the UK’s decision was not sufficiently justified, and that he did not understand the “logic” of the move – as the pandemic situations were very similar in the two countries.
He also stressed that Portugal had only registered 12 cases of the so-called Nepal variant, saying: “I don’t find any statistical relevance in these numbers.”
It comes as the coronavirus reproduction number, or R – the number of people that one infected person will pass on a virus to, on average – in England is between 1 and 1.2.
The latest figures show a further 11 people have died within 28 days of a positive coronavirus test and another 6,238 cases have been reported.
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No new destinations were added to the green list in the latest review, but seven countries – Afghanistan, Bahrain, Costa Rica, Egypt, Sri Lanka, Sudan, and Trinidad and Tobago – are being added to the red list.
Scotland, Wales and Northern Ireland have confirmed they will adopt the same changes, and the green list will be reviewed again on 28 June.
Labour has called for the amber travel list to be scrapped altogether – or “at the very least” for Thailand and Vietnam to be urgently added to the red list, citing research on a variant linked to Thailand and rising cases in Vietnam, potentially as a result of a new variant.
The travel industry has criticised the change, saying it will threaten jobs and consumer confidence – with the boss of Heathrow Airport warning the sector faces “another lost summer”.
Ryanair chief executive Michael O’Leary told BBC Breakfast that the decision was not “based on any science or public health”, and that he did not understand why the UK “which has been so successful with vaccines” was expecting its vaccinated citizens to quarantine.
media captionCoronavirus: Ryanair boss criticises ‘unnecessary disruption’ over Portugal travel
Alan Richards and familyimage copyrightFamily handout
image captionAlan Richards, centre, and his family say it is impossible for them to return home in time
Alan Richards travelled to the Algarve on Saturday with seven of his family, including grandchildren, and they were due to return home on Tuesday.
Now he says there is no possible way to get back before Portugal is moved to the amber list, costing the family hundreds of pounds in additional Covid-19 tests, keeping him from his job on the London Underground for 10 days and forcing his seven-year-old granddaughter to miss school.
“We’re not ignorant people, we understood that there may well be a situation where if anything changes you would start having to make plans to cut short your holiday. That’s acceptable,” he says.
But with just five days’ notice of the change, there is not enough time to book new pre-flight Covid tests and get the results before departure. Alan says he has been told there is a four-day wait just to get a test in Portugal.
The extra coronavirus tests they will be required to take during the 10-day quarantine will also cost £120 each, he says, adding £960 to the family’s holiday bill.
A week’s notice could have allowed them to rebook travel if necessary, Alan says.
“No matter how you look at this, the timescales for returning are absolutely awful. We’re in a frantic situation,” he says.
“Mr Shapps needs to understand the awful situation that he’s put hundreds of thousands of people in. He needs to resign.”
Portugal’s change followed increased concern about a mutation of the Delta variant first identified in India, the government said.
Some 68 cases of the Delta variant have been identified in the country, including cases with an additional so-called Nepal mutation, according to official figures.
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Public Health England (PHE) said there were cases of the mutation in several countries, including a small number in the UK, and that it was investigating whether it could be more more transmissible and less effectively tackled by vaccines.
The number of positive Covid cases in Portugal has nearly doubled in the last three weeks.
On 2 June, Portugal had 5.4 new cases per 100,000 people per day, which was only a little higher than the UK at 5.1 – but differences in the amount of testing being done make direct comparisons difficult.
Graphic showing how the traffic light system for arrivals will work
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PHE believes the Delta variant is now dominant in the UK and that it may be linked to a higher risk of hospital admission.
Earlier, epidemiologist Prof Neil Ferguson told Today that estimates suggest the Delta variant could be “60% more transmissible” than the previously dominant Kent variant known as Alpha.
Asked about whether the ending of Covid restrictions in England on 21 June needed to be delayed, he said the data was pointing “towards the direction of being cautious”.
In other key developments:
The UK has approved the use of the Pfizer-BioNTech vaccine in children aged 12-15
Cabinet Office minister Michael Gove has been alerted by the NHS Test and Trace app that he had come into contact with someone with Covid. Instead of isolating for 10 days, he will take daily Covid tests as part of a pilot scheme
The Welsh government has said outdoor concerts, festivals and sporting events can resume in Wales from Monday