The United States will require negative coronavirus test results from all air travelers arriving from the United Kingdom, the Centers for Disease Control and Prevention announced Thursday.

The test result, which should be from a PCR or Antigen test, should come no more than 72 hours before departure from the U.K. to the U.S, the CDC said.

“Airlines must confirm the negative test result for all passengers before they board,” CDC officials said. “If a passenger chooses not to take a test, the airline must deny boarding to the passenger.”

The order will be signed Friday and will take effect on Dec. 28, 2020.

“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.

The new rule comes after the U.K. announced the discovery of a new, more infectious strain of the coronavirus.

The variant, detected mainly in London and the southeast of England in recent weeks, has sparked concern worldwide because of signs that it may spread more easily.

While there is no indication it causes more serious illness, dozens of countries in Europe and beyond have restricted travel from the UK as a result.

Many of the countries swiftly halted travel last weekend after British Prime Minister Boris Johnson said London and other areas of southern England had to close all non-essential stores and call off holiday plans to curb a new, much more contagious version of the virus.

China on Thursday became the latest nation to suspend flights to and from the U.K., further isolating the country.

The CDC said air travel to the U.S. from the U.K. is already down by 90% because of travel restrictions in place since March.