The British have expressed their dismay at the prospect of Portugal returning to the UK's quarantine list – as infections cross the threshold to impose a new ban.
One vacationer said she was "losing the only thing I was looking forward to this year" as rising numbers suggest Portugal is getting back on the list – just over a week after the government said the British could travel safely there.
In the seven days ending August 30, there were 21.1 coronavirus cases per 100,000 people in the country – from 19.4 in the week ending August 29.
A rate of 20 Covid-19 cases per 100,000 people over a seven-day period is the level ministers are looking at to trigger isolation rules.
It is believed that around 75,000 UK nationals will be vacationing or flying directly to Portugal.
Many may have to fly home early to meet a deadline before a quarantine restriction is imposed.
A clampdown against travelers from Portugal would be another embarrassment for a government convicted of its "Shambolic" ranking in the travel sector during the pandemic. Quarantine rules are reviewed at the end of each week, but ministers have made it clear that they will act faster if necessary.
If the cumulative number of Covid-19 cases reaches 20 per 100,000 people over a seven-day period, ministers will consider triggering isolation rules. The total of 2,171 cases in Portugal last week gives 21.1 per 100,000 population
There are fears that Portugal could be re-quarantined by the UK after spikes in some cases brought the government dangerously close to the threshold. Pictured, Costa da Caparica beach in Almada
Searches for flights to Portugal rose after the country was removed from the UK's quarantine list just over a week ago.
Passengers coming to the UK from Portugal were no longer required to self-isolate from 4 a.m. on Saturday, August 22, after an approved travel corridor was confirmed.
The government today downplayed the prospect of routine coronavirus testing at airports – suggesting too many cases would be overlooked to be effective.
UK Ambassador to Portugal Chris Sainty said: “Travelers should carefully consider their plans and consider the risks of traveling abroad in this fast-moving situation.
"As Grant Shapps said, you only travel if you are satisfied with unexpectedly quarantining when necessary."
Environment Secretary George Eustice said border controls would always be checked but warned that screening on arrival would not eliminate the risk of introducing the disease into the country.
Chris Sainty, the UK's ambassador to Portugal, said Britons should only travel abroad if they are satisfied with an unexpected quarantine.
The comments came out of fears that Portugal could be put back on the quarantine list after a surge put the government dangerously close to the government's threshold in some cases.
In a round of interviews this morning, Mr Eustice was asked if it was time for the government to heed calls for extensive testing at airports as part of the UK's response to the pandemic.
"Obviously, all of these things are always being checked," he told Sky News.
"The previous advice for tests on arrival at airports has been that the likelihood of missing people who may be asymptomatic and therefore not appear on the test is quite high."
"A person may pass a test negative thinking they are okay and find out a few days later that they are developing symptoms."
The remarks followed concerns that nearly 200 passengers who flew from the Greek island of Zante a week ago were now being asked to self-isolate.
There are 16 cases of Covid-19 related to people who took Tui flight 6215 to Cardiff on Aug. 25.
In a round of interviews this morning, Environment Secretary George Eustice was asked whether extensive testing at airports should be part of the UK's response to the pandemic
Portugal is listed as an "amber" nation, which means it is nearing the UK quarantine threshold with 19.4 virus cases per 100,000 people over a seven-day period
Paul Charles, head of the travel advisory service PC Agency, said the figures for Portugal suggest it could be removed from the Foreign Office's list of travel corridors in a few days.
He added: "Portugal is likely to get back on the UK's quarantine list this week and the country itself is preparing an 'emergency state' from September 15.
"The company has not been able to handle its case numbers in the past two weeks as more tourists have entered Portugal, particularly Lisbon and Porto."
The PC Agency, which publishes a daily list of infection rates in each country, has listed Portugal as an "amber" nation, meaning it is nearing the UK quarantine threshold with 19.4 virus cases per 100,000 people over a seven day period – compared to 12.2 inches UK.
Britons preparing to go abroad have expressed disappointment over the possibility of a canceled vacation. One person said they were "done by 2020".
On Twitter, one user said it would be a "joke" if he were put back on the list and wrote, "I hope I don't have to cancel my vacation for the third time this year due to quarantine." Already lost money on previous cancellations. & # 39;
Vacationers are likely to be more disappointed if Portugal is put back on the quarantine list
Another complained: "The only thing I was looking forward to this year is Portugal. It was removed from the quarantine list and now it could go on again !!!" Sick and tired of everything. & # 39;
In late June, ministers began encouraging the British to vacation abroad to boost the travel industry as restrictions were eased and warned within weeks that "no travel is risk-free".
This followed the decision to introduce quarantine measures on arrivals from Spain with just five hours' notice.
Mr Charles suggested yesterday that Portugal had "been a victim of its own success in quickly attracting so many tourists".
When Portugal was added to the travel corridor list on Aug. 22 – meaning vacationers will not have to be quarantined for 14 days after returning to the UK – flight comparison site Skyscanner reported a 2,000 percent increase in bookings.