People from the UK travelling to Portugal will have to quarantine for 14 days unless they can present proof they had been fully vaccinated a fortnight before their arrival.
The Portuguese government added the UK to the list of countries from which travellers must quarantine “at home or a place indicated by the health authorities”.
The rules came into effect at midnight.
Germany also wants the European Union to restrict UK travellers.
The Times reported German Chancellor Angela Merkel wants to designate the UK as a “country of concern” because the Delta variant of the coronavirus is so widespread.
The plans will be discussed by senior European and national officials on the EU’s integrated political crisis response committee.
Mrs Merkel previously told Germany’s parliament: “In our country, if you come from Great Britain, you have to go into quarantine – and that’s not the case in every European country, and that’s what I would like to see.”
She is set to meet Prime Minister Boris Johnson at Chequers next week.
French President Emmanuel Macron has also talked about his concern at the spread of the variant.
Last week, Mr Macron said: “We should all be vigilant because the Delta variant is coming.
“We see that it affects people who have not yet been vaccinated or who have only had one dose, which means we have to be even faster in this vaccination campaign.”
Currently, fully-vaccinated UK visitors to France can enter without quarantining.
In Portugal, the legislation is in force until 11 July, but the Portuguese authorities stated it could be reviewed “at any time, depending on the evolution of the epidemiological situation”.
The new quarantine measures only apply to those travelling to mainland Portugal and not Madeira.
Brazil, South Africa, India and Nepal were already on Portugal’s quarantine list, but the exception for people who are vaccinated against Covid-19 to avoid isolating only applies to the UK.
For all countries on the list, an exception is also made for participants in specified sporting competitions being held in June and July.