LONDON — Britain has removed Portugal from its list of coronavirus-safe travel destinations, meaning thousands of U.K. residents currently on vacation there will have to quarantine on return.

Transport Secretary Grant Shapps says the “difficult decision” was prompted by rising case rates in Portugal and worries about new strains of the virus that could prove resistant to vaccines. The change will take effect Tuesday.

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

Tourism is a mainstay of Portugal’s economy, accounting for about 15% of annual gross domestic product.