Britain said Thursday that it is removing Portugal from its list of COVID-safe travel destinations, meaning thousands of U.K. residents currently on vacation there face the prospect of 10 days’ quarantine on return.

Transport Secretary Grant Shapps said the “difficult decision” was prompted by rising infection rates in Portugal and worries about new strains of the virus that could prove resistant to vaccines.

Shapps said, “There’s a sort of Nepal mutation of the so-called Indian variant which has been detected and we just don’t know the potential for that to be a vaccine-defeating mutation and simply don’t want to take the risk as we come up to June 21” the date the U.K. government hopes to lift remaining coronavirus restrictions.

The change will take effect at 4 a.m. on Tuesday. Territories including Iceland, Israel and the Falkland Islands remain on the U.K. green list.

The U.K. has recorded almost 128,000 coronavirus deaths, the highest toll in Europe. A mass vaccination campaign that started in December has brought new infections and deaths down sharply, but case numbers are once again rising as a more transmissible virus variant, named Delta by the World Health Organization and first identified in India, spreads across the U.K.

Portugal is a major destination for sun-seeking Britons, and was the only large tourism destination on the U.K. government’s “green list,” announced last month, of places that can be visited without the need to self-isolate on return.

Tourism, by mainly British visitors, is a mainstay of the southern European country’s economy, accounting for around 15% of annual gross domestic product.

Britain also is adding seven countries, including Sri Lanka and Egypt, to its “red list” of places with severe COVID-19 outbreaks to which all but essential travel is barred. U.K. residents returning from red list countries must spend 10 days in a government-approved quarantine hotel.

Travellers returning from dozens of “amber list” countries, including the United States, must complete a 10-day quarantine at home.