Germany is banning most travel from Britain starting on Sunday amid concerns about the spread of a coronavirus variant first discovered in India, the German authorities said on Friday.

German citizens and residents of Germany will still be allowed to enter the country from Britain but will be required to self-isolate for two weeks upon arrival, Germany’s public health institution said as it classified Britain as an area of concern because of the variant.

The move came just days after Britain reopened its museums and cinemas and resumed allowing indoor service in pubs and restaurants. Many people in Britain have been looking forward to traveling abroad in the coming months, and Spain is set to welcome visitors arriving from Britain without a coronavirus test starting on Monday.

The spread in Britain of the variant first detected in India, known as B.1.617, could serve as an early warning for other European countries that have relaxed restrictions. This month, the World Health Organization declared the mutation a “variant of concern,” and although scientists’ knowledge about it remains limited, it is believed to be more transmissible than the virus’s initial form.

Brazil, India and South Africa are among the dozen or so other countries that Germany considers areas of concern because of variants. As of Thursday, Britain had 3,424 cases of the variant first discovered in India, according to government data, up from 1,313 cases the previous week.

Dozens of nations, including European countries and the United States, suspended travel from Britain or imposed strict restrictions earlier in the pandemic amid concerns about the spread of a variant first detected in England.

Britain’s Office for National Statistics said on Friday that the percentage of people testing positive for the coronavirus in England had showed “early signs of a potential increase” in the week ending May 15, although it said rates remained low compared with earlier this year. At its peak in late December, Britain recorded more than active 81,000 cases, compared with about 2,000 this month.

The country’s inoculation campaign is continuing apace, with an increased focus on second doses in an effort to thwart the sort of spikes that led to restrictions imposed earlier this year.

More than 37 million people have received a first dose of a Covid-19 vaccine in Britain — 56 percent of the population. Yet most people under 30 have yet to receive a dose, and less than a third of the population has been fully vaccinated. Health Minister Matt Hancock said on Saturday that people over 32 could now book an appointment.

Prime Minister Boris Johnson has vowed to proceed with a plan to lift all restrictions by June 21, although scientists have warned that the spread of the B.1.617 variant could delay such plans. Most cases of the variant have been found in northwestern England, with some in London.

In Germany, the restrictions on travel from Britain come as outdoor service resumed on Friday in cafes, restaurants and beer gardens after months of closure. Chancellor Angela Merkel urged people to “treat these opportunities very responsibly.”

“The virus,” she said, “has not disappeared.”