The new rule, which takes effect on Monday, will apply to Americans as well as foreign nationals.
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 on 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 on 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 P.C.R., 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 C.D.C. 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 C.D.C. said, adding that “if a passenger chooses not to take a test, the airline must deny boarding to the passenger.”
The new rules were a reversal for the Trump administration, which initially told American 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.
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 on Saturday, new restrictions further limit socializing, and schools and universities might soon have to close.
The British Foreign Office updated its travel advice online on Friday to warn travelers of the new testing requirement. “We are in close contact with U.S. authorities and working urgently to minimize disruption as far as possible,” the office said in a statement. “British travelers should follow the U.S. authorities’ guidance, and speak to their airlines for the latest travel options in the first instance.”
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 P.C.R. 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 P.C.R. 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.
Several airlines had already announced policies requiring proof of a negative test after a demand from Gov. Andrew M. Cuomo of New York that passengers arriving from London to John F. Kennedy International Airport would need to provide documentation of a negative test result.
“We can’t let history repeat itself with this new variant,” Mr. Cuomo had written on Twitter.
Also on Thursday, Gov. Phil Murphy of New Jersey said that passengers arriving at Newark Airport would need negative tests within 72 hours of departure to enter.
The American travel 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 S. Fauci, the top U.S. 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 U.K. here.”
Over 4.8 million British residents visited the United States in 2019, according to Britain’s Office of National Statistics. Earlier this year, Heathrow Airport said that over 1 million passengers were attributed to the North American market in the month of February. Since then, according to data compiled by the airport, the figure has plummeted, to just 81,713 in November.
A recent study by British scientists found no evidence that the variant was more deadly than others. But the researchers estimated that it was 56 percent more contagious. The country also announced a ban on travel from South Africa after the health secretary, Matt Hancock, said two people had been discovered with another variant that emerged in the African country. Another variant has also emerged in Nigeria.
But concerns over the variant still led governments to impose restrictions on travel from Britain. A French ban on both travelers and freight led to blockages at the port of Dover for 48 hours, leaving thousands of truck drivers stranded even as the ban was lifted on Wednesday.
Vivian Wang contributed reporting.
