Image copyright
Getty Images
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 who 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 guidance comes as infection rates begin to change across Europe, with Spain seeing a spike in cases in popular holiday resorts.
Portugal's failure to make the exemption list will come as a major blow.
Tourism is a major industry in the country 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.
'Huge uncertainty'
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."