Changing an airline ticket is getting more expensive at US Airways and United Airlines.

Last week United quietly boosted the fee to change a ticket in advance to $200, from $150. A spokesman for United Continental Holdings Inc. says the fee helps cover the cost to the airline when a traveler gives up a reserved seat.

On Wednesday US Airways did the same thing. The website for US Airways Group Inc. also shows that the airline now charges up to $300 to change some international tickets.

The airlines also make travelers pay the higher fare if they switch to a more expensive flight.

Fees for changing tickets and for baggage have become a major source of revenue for the biggest U.S. airlines, although Southwest doesn’t charge to change a ticket.