LISBON - Passengers arriving in Portugal from Britain must quarantine for 14 days from Monday if they are not fully vaccinated against COVID-19, the Portuguese government said.

The new rule will remain in place until at least July 11. It follows a surge in cases in Portugal to levels seen in February, when it was under a strict lockdown.

Daily positive cases have also risen in Britain but the vaccination roll-out has been faster there, and a much higher proportion of adult Britons have received a first dose.

Britons arriving 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," the government said in a statement late on Sunday.

A person is considered fully vaccinated in Portugal 14 days after their second of two vaccinations or the Johnson & Johnson single-dose vaccine. Passengers from Britain who have recovered from COVID-19 and received one dose will also be allowed in.

Britain is one of Portugal's biggest sources of foreign tourists but removed Portugal from its quarantine-free travel list this month. Germany declared Portugal a "virus-variant zone" last week, a measure that will further disrupt travel. 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 rise in infections on the more contagious Delta variant, first identified in India but spreading in Portugal and Britain.

It accounts for over 70% of cases in the Lisbon area and is spreading to other parts of the country, which has the EU's second highest seven-day rolling average of cases per capita, according to online publication Our World in Data. 

Portugal opened borders to British tourists in mid-May and thousands of English soccer fans arrived for the Champions League final last month.

(Reporting by Catarina Demony; Additional reporting by Philip Blenkinsop; Editing by Kirsten Donovan and Timothy Heritage) ((catarina.demony@thomsonreuters.com; +351 915 616 444 (Twitter: @CatarinaDemony);))