From Monday, unvaccinated passengers arriving in Portugal from Britain must quarantine for 14 days.
That's according to a Portuguese government statement issued late on Sunday.
The new rule, which will remain in place until at least July 11, comes amid a surge in cases in Portugal, where the number of daily new COVID-19 infections is back to February levels, when the country of just over 10 million was still under a strict lockdown.
Daily positive cases have also been rising in Britain for a month. Britons arriving in Portugal by air, land or sea must show proof they are fully vaccinated or self isolate for 14 days at "home or at a place indicated by health authorities."
Britain, one Portugal's biggest sources of foreign tourists, removed the southern European destination from its quarantine-free travel list earlier this month.
Portugal is on Britain's amber list, meaning holidaymakers must self-isolate for 10 days when they return home and take expensive COVID-19 tests.
Portuguese health authorities have blamed the more contagious Delta variant, first identified in India but rapidly spreading in Britain too, for the recent rise in infections.