United Airlines will begin charging some passengers $50 to check in a second piece of luggage on domestic round-trip flights, becoming the first big carrier to impose a fee for a service that has long been included in the price of a ticket.

As of late Monday no other major carrier had followed United, but some analysts said that if the move didn't generate significant resistance from consumers, the traditional two-free-bag rule was likely to go the way of other amenities such as free meals and pillows.

"Everybody is chiseling away at everything that you thought you deserve," said Tom Parsons, chief executive of the Bestfares.com travel website. "But people shouldn't be upset because we still want to fly coast to coast for $199."

United, the nation's second-largest airline and one of the busiest at Los Angeles International Airport, cited higher fuel costs for the new fee and said that only about 25% of its passengers check a second bag.

The fee -- $25 for the second bag each way -- is expected to generate more than $100 million in additional revenue. United also expects fewer checked bags as a result of the fee, which could lower its fuel expenses.

United's chief revenue officer, John Tague, announced the new policy Monday, saying the fee would allow the airline to "offer competitive fares to everyone."

The move to charge for luggage would bring so-called legacy carriers one step closer to ultra-low-cost airlines, which offer tickets as low as $20 for a round-trip flight but charge for everything, including drinks, pillows and checked luggage.

Columbus, Ohio-based Skybus Airlines and Spirit Airlines, headquartered in Miramar, Fla., charge $5 per checked bag each way up to two pieces. Later this month, Spirit will double the baggage fee to $10 for flights booked on the Internet. If a passenger pays at the airport, the baggage fee will go up to $20.

"There are all kinds of fees these days and rules are changing by the minute, so it's flier beware," Parsons said.

United's fee, which would take effect for flights beginning May 5, is expected to hit families and leisure travelers the hardest.

The policy doesn't apply to international flights or to passengers who buy higher-priced refundable tickets or those who have "elite" status with the airline's frequent-flier programs. They will still be able to check two bags for free.

But for families on a budget who have purchased discounted, nonrefundable tickets, the fees are likely to be hefty because many typically check two bags per family member.

Tony Bergman, a Brea resident who was picking up his mother-in-law at Orange County's John Wayne Airport on Monday, said he would have to pay $150 in fees to check two bags each for himself, his wife and his 4-year-old daughter.

That would be on top of the new fuel surcharges that can add $40 to the cost of a ticket, plus fees for curbside check-in as well as meals that would have to be purchased on board.

"People are getting tired of being nickeled and dimed," Bergman said as his mother-in-law, Maureen Notz, who was visiting from Bentonville, Ark., became increasingly upset.

"It's ridiculous," Notz said as she waited to claim her two pieces of luggage from a United flight that connected through Denver. "It's like the airline is saying, 'Let's get the middle-class people.' I hope nobody else does what United is doing."