LISBON (REUTERS) - Portugal's prime minister criticised Britain on Sunday (June 6) for removing his nation from a Covid-19 quarantine-free travel list, and urged London to adhere to a European digital certificate scheme to ease travel.

Britain said last week it was removing Portugal from its "green list" of countries that do not require quarantine on return because of rising Covid-19 case numbers and the risk posed by coronavirus variants detected in Portugal.

Portugal had been placed on the "green list" just weeks earlier. But from 0400 GMT on Tuesday, Britons returning from Portugal will need to quarantine for 10 days and take two Covid-19 tests.

"We can't have this system of instability and changes every three weeks," Prime Minister Antonio Costa told reporters. "It isn't good for those who plan their holidays, nor for those who have to organise the tourism industry to receive tourists in good conditions."

He said Portugal was maintaining dialogue with the British government to "explain the decision is not justified and also the serious damage it causes to the British and to the Portuguese economy."

Costa said "a good way for the British to find a solution to this situation" would be to adhere to a system of digital certificates which the European Union plans to introduce from July 1.

Under this scheme, EU citizens who can prove they have been vaccinated against Covid-19, tested negative for the disease or recovered from it will be able to travel freely in the EU.

Britain, however, has left the EU.

Costa did not explain how it might adhere to the scheme.

New coronavirus cases have increased slightly in Portugal since it began lifting lockdown restrictions last month.

Hospitalisations are low and daily deaths are near zero following the vaccination of nearly all its most vulnerable elderly citizens.