Britain’s ambassador to Portugal has said the new air bridge to Portugal could “change quickly”, as holidaymakers scramble to return before an expected reimposition of quarantine restrictions.

Chris Sainty fuelled speculation on Tuesday that the restrictions for those travelling back from Portugal could be reintroduced later this week, only two weeks after they were lifted.

Before a weekly meeting on Thursday to review the travel corridor list, Sainty advised potential visitors to Portugal to only travel “if you are content to unexpectedly quarantine if required”. He also reminded travellers that “things can change quickly”.

Sainty’s remarks came after the number of coronavirus cases in Portugal increased above 20 per 100,000 people, the level at which the UK government considers triggering quarantine conditions that require arrivals to self-isolate for 14 days.

There were 22.3 coronavirus cases per 100,000 people in Portugal in the seven days to 31 August, up from 14.5 a week earlier. The seven-day rate in the UK is 13.9.

If it is decided at Thursday’s meeting to remove Portugal from the list of safe countries, those returning from the country can expect quarantine to be imposed from Saturday at 4am.

EasyJet has sold out all its flights from Faro, which serves the Algarve, to airports in Britain on Wednesday, Thursday and Friday.

British Airways is selling seats on a flight from Faro to Heathrow on Thursday for £554. The same journey can be made seven days later for £139.

The head of British Airways’ parent company has said that reimposing quarantine for people arriving from Portugal would cause further “chaos and hardship” for travellers.

Writing in the Times, Willie Walsh, the chief executive of International Airlines Group, claimed the “ever-shifting list” means “the UK has officially hung up the ‘closed’ sign”.

He added: “The government is using arbitrary statistics to effectively ban 160 countries and in the process destroying the economy. The government needs to introduce a testing regime to restore confidence.”

Despite the risk of quarantine being reimposed, Visit Portugal, an initiative by the Portuguese government to boost tourism, is still encouraging UK holidaymakers to travel.

Its English language Twitter feed on Tuesday tweeted: “#Algarve a go-go! … The air bridge to Portugal is up, and that means the joys of the Algarve are available once more to British holidaymakers.”

There was no warning that quarantine could be reintroduced as early as this weekend.

In June, the Portuguese foreign minister, Augusto Santos Silva, complained that “quarantine is an enemy of tourism”.