US travellers on planes and public transport will be required to wear face masks from Tuesday to curb the spread of COVID-19.
The US Centers for Disease Control and Prevention issued the rule late on Friday, building on an order announced on January 21 by President Joe Biden.
The rule “will protect Americans and provide confidence that we can once again travel safely even during this pandemic”, said Dr Marty Cetron, director of CDC’s division of migration and quarantine.
The 11-page order takes effect just before midnight on Monday.
It makes refusal to wear a mask a violation of federal law, enforced by the Transportation Security Administration and other federal, state and local authorities.
The rule applies to passengers on planes, trains, subways, buses, taxis and ride-shares.
It says travellers must wear a mask that covers their nose and mouth while riding and while getting on and off rides.
The order extends to waiting areas such as airports, train platforms and subway stations.
Airlines already require masks and have banned more than 2000 passengers for refusing to wear one.
Flight attendant unions say the federal rule will make it easier for crews to enforce the requirement.
The order exempts children under 2 years of age and people with a disability that makes it unsafe to wear a mask.
The CDC said transportation operators may require a negative COVID-19 test from passengers.
Cetron said this week the agency is considering requiring testing of passengers on flights within the United States but the airline industry is fighting it out of fear fewer people will fly.
US air travel is already down more than 60 per cent from a year ago.