New York Times – The United States will require all airline passengers arriving from Britain to test negative for the coronavirus within 72 hours of their departure, the Centers for Disease Control and Prevention said Thursday.

The move comes as a new highly transmissible variant of the virus, which first appeared in Britain, has led more than 50 countries to seal their borders to travelers from there or to impose restrictions on their arrival.

The new rule, which takes effect Monday, will apply to Americans as well as foreign citizens and will require passengers to show proof of a negative result on a genetic test, known as a PCR, or an antigen test.

“This additional testing requirement will fortify our protection of the American public to improve their health and safety and ensure responsible international travel,’’ the CDC said in a statement.

Passengers will be required to “provide written documentation of their laboratory test result (in hard copy or electronic) to the airline,’’ the CDC said, adding that “if a passenger chooses not to take a test, the airline must deny boarding to the passenger.’’

The rules were a reversal for the Trump administration, which initially told US airliners that the government would not require testing for travelers from Britain.

United Airlines, Virgin Atlantic, and Delta Air Lines had already announced similar policies, requiring all passengers on their flights between Britain and the United States to submit proof of a negative test result within 72 hours of departure. British Airways had also been requiring negative test results for passengers arriving in New York.

At Logan International Airport, about 10 flights a week arrive from London on British Airways and Virgin Atlantic planes.

The announcement from the United States adds another layer of difficulty for Britons hoping to travel. Nonessential travel will also be banned within much of Britain starting Saturday, new restrictions further limit socializing, and schools and universities might soon have to close.

The British Foreign Office updated its travel advice online Friday to warn travelers of the new testing requirement. “We are in close contact with US authorities and working urgently to minimize disruption as far as possible,’’ the office said in a statement.

People traveling immediately after the holiday may face uncertainty: Many private testing clinics and labs are closed on Christmas Day, so testing within the 72-hour window may prove difficult, especially for the PCR screening, which must be sent to a lab and can take several days to process.

The rapid antigen test, a relatively new tool to detect the virus, gives a result in around 30 minutes, but it is not as widely available, although it is cheaper. Heathrow Airport, for example, charges passengers about $130 for PCR results with 48 hours and about $60 for antigen tests with results within 45 minutes.

Both tests are offered at major British airports — including Heathrow and Gatwick, London’s two major hubs, and Manchester Airport — but passengers must register in advance. It was unclear how many would be able to procure a test and get a result in time for travel.

The introduction of new travel restrictions led to concerns that travelers to the United States would flock to the airport, as Londoners did at train stations last Saturday when tighter domestic rules were announced. But employees at Heathrow on Friday described a normal, if quieter, stream of passengers typical of Christmas Day, with most appearing to travel on long-haul flights.

The US requirements are less Draconian than those of other countries in Europe and Asia, which barred all travelers from Britain after the new coronavirus variant emerged. Experts are skeptical that travel bans can stop the spread of the variant. In fact, Dr. Anthony Fauci, the top US infectious disease expert, said there was a good chance that the variant was already in the country.

“I don’t think that that kind of a Draconian approach is necessary,’’ he said of a travel ban on “PBS NewsHour.’’ “I think we should seriously consider the possibility of requiring testing of people before they come from the UK here.’’

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