Putting Portugal back on the UK's quarantine list would cause further "chaos and hardship" for travellers, the boss of British Airways' parent company has warned.
Quarantine measures could be reimposed on British holidaymakers returning from Portugal due to a rise in Covid-19 cases.
There were 21.1 coronavirus cases per 100,000 people in Portugal in the seven days to August 30, up from 19.4 in the seven days to August 29.
A seven-day rate of 20 is the threshold above which the UK Government considers triggering quarantine conditions.
Searches for flights to Portugal soared after the country was removed from the UK's quarantine list less than a fortnight ago.
Passengers arriving in the UK from Portugal no longer had to self-isolate from 4am on Saturday August 22 after an approved travel corridor was confirmed.
Writing in The Times, Willie Walsh, chief executive of International Airlines Group, said: "Another U-turn by the Government, adding Portugal to the quarantine list, will cause further chaos and hardship for travellers."
He said the "ever-shifting list" of countries requiring quarantine means "the UK has officially hung up the 'Closed' sign".
Mr Walsh said: "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."
Google search data showed a significant spike in searches for the term "flights to Portugal" by users in the UK at around 6pm on Thursday August 20, the day the lifting of the quarantine was announced.
A number of easyJet flights from London airports to destinations across Portugal on the weekend of Saturday August 22 were already unavailable on the Friday.
Jet2 was among the airlines aiming to capitalise on the rush, adding extra seats to Faro from Monday August 24 from across the UK.
Travellers returning from Switzerland, Jamaica and the Czech Republic were the latest to be told they must quarantine for 14 days if they were arriving in the UK after 4am on Saturday following spikes in coronavirus infection rates.