New York ? United Airlines plans to charge certain domestic passengers for more than one piece of checked-in luggage to help offset the soaring cost of jet fuel, the No. 2 U.S. carrier said Monday.

The move by the operating subsidiary of UAL Corp. is the first taken among the so-called legacy carriers including Northwest Airlines, Delta Air Lines and Continental Airlines, but it’s bound to be imitated as a way to defray expenses without alienating fare-conscious travelers.

“They are the first, but they certainly won’t be the last,” said airline consultant Terry Trippler of Trippler Associates. For the airlines, each bag is additional weight and thus represents fuel expense. “The airlines are looking for every possible revenue source they can find and still cut fares without cutting wages, because right now they can’t go any lower.”

For domestic passengers who do not have status in a Mileage Plus or Star Alliance, a second piece of checked luggage will cost them $25, effective May 5. There will be no charge for their first bag.

Airlines across the industry have struggled to deal with soaring jet fuel prices while remaining competitive. Chicago-based United said the new policy is expected to generate $100 million in annual cost savings and new revenue.

In the quarter ended Dec. 31, UAL Corp. said fuel costs increased $359 million, or 25 percent, from a year earlier.

Some smaller airlines already have been charging passengers for checking in more than one piece of luggage, while at least one low-cost carrier, Spirit Airlines, charges $10 each for the first two bags.

United said that, based on research into passengers’ habits, it determined that only one in four check in more than one bag. By charging the 25 percent of people who check in more than one bag, the airline said it can bring down the weight of the airplane and help keep fares in check.