WASHINGTON, Nov 20 (Reuters) - A U.S. Senate panel on Monday announced an investigation into airline fees for baggage, seat selection, ticket changes and other services, demanding justifications from the CEOs of five major carriers for these charges that generate billions of dollars in annual revenue for them.

Democratic Senator Richard Blumenthal, who chairs the Permanent Subcommittee on Investigations, said these fees are often hidden from and confusing to consumers.

Blumenthal said he wrote the chief executives of American Airlines, United Airlines, Delta Air Lines , Spirit Airlines and Frontier Airlines seeking a detailed breakdown on how much they collect from each fee, the reason for them and the costs to supply each service.

Between 2018 and 2022, total revenue across major U.S. airlines from baggage fees increased from $4.9 billion to $6.8 billion, the senator said. Blumenthal also cited a report by a travel consultancy that found that eight leading U.S. airlines last year collected an estimated $4.2 billion in fees for seat selection.

"U.S. airlines increasingly charge ancillary fees that obscure the actual cost of air travel," Blumenthal said in the letters.

"These itemized fees are often not disclosed to customers until well into the ticket purchasing process or after a ticket has been purchased, making it difficult for customers to know the true, total cost of a ticket and comparison shop prior to purchase," Blumenthal added.

American, Delta and United referred questions about the Senate investigation to Airlines for America, an industry trade group, which did not immediately respond to a request for comment. Spirit and Frontier did not immediately respond to requests for comment.

Airline CEOs in 2018 lobbied against bipartisan legislation to mandate "reasonable and proportional" baggage and change fees and convinced Congress to drop the plan.

The U.S. Transportation Department last year proposed requiring airlines to disclose fees for baggage, ticket changes and family seating the first time an airfare is displayed. It also proposed rules in 2021 to require airlines to refund fees for significantly delayed bags and refunds for services like onboard Wi-Fi that do not work.

The department is scheduled to finalize both of those regulations in early 2024.

Aviation legislation has stalled in Congress that among other things would bar airlines from charging families with young children from sitting together in most instances. (Reporting by David Shepardson Editing by Will Dunham and Chris Reese)