Portugal remains off the list of countries that the government has exempted from quarantine restrictions.
In changes that apply to England, travellers from Estonia, Latvia, Slovakia, Slovenia and St Vincent and the Grenadines won't have to isolate.
It takes the list of countries that do not face travel restrictions into England to 80 nations.
The government also said it will update guidance weekly, meaning rules could change while people are away.
It said people should regularly check the advice. Previously, updates were provided every three weeks.
The Portuguese government expressed "regret" at the UK's decision to continue to exclude it from the list of countries that are exempt from quarantine.
"It is a decision that is neither substantiated nor supported by the facts," said Portugal's Ministry of State and Foreign Affairs.
Spain remains on the list of countries that people can return to England from without the need to self-isolate, despite a recent spike in coronavirus cases.
On Friday, Norway announced that it was imposing a new 10-day quarantine on all travellers arriving from Spain.
According to Johns Hopkins University, Spain has recorded270,166 cases of coronavirus and28,429 deaths.
Portugal had recently imposed local lockdowns on the outskirts of its capital Lisbon to stem a rise in new cases.
Portugal has recorded 49,379 coronavirus cases and 1,705 deaths.
Tourism is a major industry in Portugal and is popular with British holidaymakers, with almost three million UK visitors a year.
Aviation data analysts Cirium said there were 2,333 flights due to leave the UK for Portugal before the end of August.
Paul Charles, chief executive of the PC Agency, said it was a badly timed move by the government.
"The scale of those due to go there before end of August is enormous. The decision today plants huge uncertainty in the minds of those who are booked who will be looking for refunds and changes and most won't have a holiday. It's going to cause uproar for operators and industry."
He added: "They are not prepared to open Portugal when situation is declining, but cases in Spain are soaring, with rapid rises in their case numbers."
Joe Mountain of Sandy Blue, a villa rental firm in the Algarve, told the BBC that he is deeply worried about the impact of a sharp drop in tourism on the local industry.
"The decision is absolutely ridiculous. If you look at the Algarve specifically, we've got exceptionally low rates of coronavirus here. There's only been, as of today's count, 806 [cases]," he said.
"The Algarve absolutely depends on tourism - from hotels, to restaurants, to bars, and there will literally be thousands of job losses on the back of this."