Mainland Portugal appears increasingly likely to be removed from the government’s quarantine-exemption list.
The country reported 646 new coronavirus cases on Wednesday, which is its highest daily total since late April.
This means Portugal’s seven-day rate of cases per 100,000 people is 28.3.
A seven-day rate of 20 is the threshold above which the UK Government considers triggering quarantine conditions.
Changes to the government’s list of countries from which arrivals in England do not need to self-isolate for 14 days are expected to be announced on Thursday and implemented at 4am on Saturday.
That means UK holidaymakers face a scramble to return home before restrictions come into force.
British Airways flights from Faro – which serves Portugal’s Algarve region – to London on Friday cost £307.
EasyJet’s flights departing from the same airport to Gatwick are sold out, but it is selling seats on a flight to Luton for £124.
Portuguese islands Madeira and the Azores could retain their quarantine-free status.
On Monday, the Department for Transport launched a new islands policy for travel corridors, which saw mainland Greece keep its exemption but seven islands have restrictions reimposed.
On Thursday, Transport Secretary Grant Shapps said: “We’ll be able to look at islands in other places, notably the Portugal and Spanish islands.
“But whether their data justifies (exempting them from quarantine requirements) at the moment is something I’m afraid we’ll have to wait until later today when we’ve completed our discussions with the Joint Biosecurity Centre.”
Denmark could also lose its exemption as its current seven-day rate is 24.3.
The rate is 24.8 in the UK and 21.4 in Ireland.
Figures have been calculated by the PA news agency based on data collected by the European Centre for Disease Prevention and Control.
Meanwhile, more than a dozen airline leaders have written to Boris Johnson, warning that the islands policy is “by no means enough to support the sector”.
The letter, whose signatories include the chief executives of easyJet, Ryanair, British Airways and industry association Airlines UK, called for airport testing to be implemented before the end of September as a way of reducing self-isolation periods to five days.
It asked the prime minister to begin trialling tests on arrival and after five days on a route such as London-New York “to give ministers reassurance that a one-test-on-arrival system is safe”.
The airline leaders want certain mainland regions of countries to be added to the quarantine-exemption list.
They also asked Mr Johnson to waive air passenger duty for 12 months and extend the Job Retention Scheme for aviation workers.
The letter warned that “our industry is in crisis” and the UK is “falling well behind international rivals”.
It concluded: “We ask you to act urgently to implement a programme of recovery for our sector.”