U.S. airlines are suing to block the Biden administration from requiring greater transparency over fees that the carriers charge their passengers, saying that a new rule would confuse consumers by giving them too much information during the ticket-buying process.

The U.S. Transportation Department said Monday it will vigorously defend the rule against what it called “hidden junk fees.”

American, Delta, United and three other carriers, along with their industry trade group, sued the Transportation Department in a federal appeals court on Friday, asking the court to overturn the rule.

The trade group, Airlines for America, said the Transportation Department is going beyond its authority by attempting “to regulate private business operations in a thriving marketplace.” The airlines said the administration hasn’t shown that consumers can’t get information about fees already.

“Airlines go to great lengths to make their customers knowledgeable about these fees,” Airlines for America said Monday. “The ancillary fee rule by the Department of Transportation will greatly confuse consumers who will be inundated with information that will only serve to complicate the buying process.”

The Transportation Department announced the new rule on April 24. It would require airlines and travel agents to disclose upfront any charges for baggage and canceling or changing a reservation. Airlines must show the fees on the first website page where they quote a price for a flight.

The agency estimated that the rule will save consumers more than $500 million a year.

“We will vigorously defend our rule protecting people from hidden junk fees and ensuring travelers can see the full price of a flight before they purchase a ticket. Many air travelers will be disappointed to learn that the airline lobby is suing to stop these common-sense protections,” the department said Monday.

Among the nation's six biggest airlines, only Southwest did not join the legal challenge, which was filed in the 5th U.S. Circuit Court of Appeals in New Orleans. Southwest said the rule will have little to no effect on it because the Dallas-based carrier lets passengers check two bags for free and has never charged extra fees for changing or canceling reservations.

“Overall, we support every airline’s right to price its products but believe fees should be clearly and consistently disclosed, so consumers can make informed purchasing decisions,” Southwest said.