From Tuesday, only citizens and residents of Germany will be allowed in from the two countries, with air, rail and bus companies prohibited from transporting other passengers from these places.
Those allowed to enter from Russia and Portugal will be subject to a two-week quarantine, regardless of whether they are vaccinated or have tested negatively for Covid-19. There is no option to end the quarantine early with a negative test.
Both countries are currently reporting a surge in cases of the Delta variant.
A total of 14 other countries have already been placed in Germany’s highest-risk “virus variant area of concern” category.
They include Britain, India, South Africa and Brazil, where highly transmissible strains of Covid-19 are raging.
German health authorities have issued repeated warnings about the Delta variant, which was first detected in India and which is circulating widely in Britain.
It comes as Germany has been opening its borders up to some countries, including the US, in a bid to revive tourism.
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Robert Wieler, who heads the disease control agency RKI, has said the Delta variant is expected to become the dominant strain in Germany by the autumn.
German Chancellor Angela Merkel and French President Emmanuel Macron on Thursday urged fellow EU leaders to take a firm line on travel from Britain to prevent the variant spreading in the bloc.
“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,” Merkel said earlier this week.