WASHINGTON, Oct 3 (Reuters) - The U.S. Federal Aviation Administration (FAA) is set as early as Tuesday to require that airline flight attendants receive at least 10 hours of rest time between shifts, an action that Congress directed in 2018, sources told Reuters.

The FAA first proposed the rule in October 2021. Under existing rules, flight attendants get at least nine hours of rest time but it can be as little as eight hours in certain circumstances. The FAA declined to comment on Monday.

Airlines for America, a trade group representing major carriers including American Airlines, Delta Air Lines , United Airlines and others, had previously estimated the rule would cost its members $786 million over 10 years for the 66% of U.S. flight attendants its members employ, resulting from things like unpaid idle time away from home and schedule disruptions.

Aviation unions previously told the FAA the majority of U.S. flight attendants typically do receive 10 hours of rest from airlines but urged the rule's quick adoption for safety and security reasons.

Last year, the FAA cited reports about the "potential for fatigue to be associated with poor performance of safety and security related tasks," including in 2017, when a flight attendant reported almost causing the gate agent to deploy an emergency exit slide, which was attributed to fatigue and other issues.

The FAA estimated last year the regulation could prompt the industry to hire another 1,042 flight attendants and cost $118 million annually. If hiring assumptions were cut in half, it said, that would cut estimated costs by over 30%.

After the FAA published an advance notice of the planned rules in 2019, Delta announce it would mandate the 10-hour rest requirement.

Sara Nelson, president of the Association of Flight Attendants representing 50,000 workers at 17 airlines, said last year the rule was critical.

"Congress mandated 10 hours irreducible rest in October 2018, but the prior administration put the rule on a process to kill it." (Reporting by David Shepardson; Editing by Leslie Adler and David Gregorio)