The US Federal Government is to appeal against a court ruling that struck down the mandatory wearing of face masks on airlines and public transport systems.
Since Tuesday, it has no longer been a legal requirement to wear face masks on shared transport in the USA.
However, the Centers for Disease Control, the federal agency in charge of combating public health threats, wanted to reinstate its legal power to issue mask mandates.
On Monday, a federal judge in Florida ruled the CDC was acting beyond its legal powers in imposing mandatory mask wearing on airlines and public transport systems.
The ruling had immediate nationwide effect.
On Tuesday morning, airlines, train companies and bus and taxi operators dropped the requirement for passengers to wear masks.
US President Joe Biden said passengers decision to wear masks "is up to them".
But last night the CDC asked the federal government to mount an appeal against the Florida ruling, saying mask wearing was still needed to control the spread of Covid.
The agency is also concerned that it could lose the right to impose restrictions in the face of a new pandemic in the future.
CDC says mask wearing 'remains necessary'
In a statement, the CDC said it was the agency's "continuing assessment that at this time an order requiring masking in the indoor transportation corridor remains necessary for the public health. CDC believes this is a lawful order, well within CDC’s legal authority to protect public health".
The ruling appears to have caught the Biden administration off guard and provoked days of debate on how to respond.
The sudden change of policy from the court ruling, and the Government's late decision to appeal left many passengers confused.
The appeal process may add to that confusion, particularly if the federal government requests the appeal court put a stay on the Florida ruling, to allow it to re-instate mandatory mask wearing while the appeal takes place.
The argument may progress all the way through the federal legal system to the US Supreme Court.
Airline companies in the US have been lobbying hard to have the mask mandate removed from air passengers.
The passenger train company Amtrak told passengers it was no longer a legal requirement to wear masks, but said it would prefer if they did.
The ride sharing company Uber still asks passengers and drivers to wear masks as a condition of carriage.
City governments in New York and Philadelphia say their own local mask mandates on public transport remain in force, and are unaffected by the Florida ruling.
Mandatory mask wearing and other legal requirements to control Covid have become politically divisive in America, with opinion polls showing most Republican voters opposing the measures, while most Democrat voters support them.