Millions of passengers soon will have to pay extra to check a second bag on United Airlines.

Citing the extraordinary cost of jet fuel, United yesterday became the first major U.S. carrier to impose a $25 fee each way on domestic passengers who buy the cheapest tickets and check two suitcases. Checking a third bag will cost an additional $100 each way.

United carries more people between Hawai'i and the Mainland than any other airline.

Until now, checking two bags on flights has been free on United and every other major carrier.

But luggage is heavy, and extra weight requires more fuel to transport. In the October-December quarter, fuel cost United $1.4 billion, 26 percent more than a year earlier. Fuel is airlines' largest expense, eclipsing wages.

"The passenger who checks two bags should pay the extra cost of carrying two bags," said Robin Urbanski, spokeswoman for the Chicago-based carrier.

The new bag fees will apply to United and United Express passengers who are not elite members of United's Mileage Plus frequent-flier program, and who book nonrefundable economy-class tickets for trips within the United States and between the U.S. and Canada. Elite status requires members to log at least 25,000 miles a year.

United's bag fees will apply to travel starting May 5. Passengers will be able to pay the fee with a credit card at check-in kiosks or with cash or a credit card at check-in counters.

Other major carriers said yesterday they are studying United's move and would not say whether they will follow.

"United is the first to do it, but I don't think it will be the last," said airline expert Terry Trippler, CEO of Web site Trippler Travel.com. "I think charging for sodas is next. Airlines are looking for every possible penny of revenue they can find. They can't cut wages any more."

Airlines this year already have raised fares and imposed fuel surcharges to help cover skyrocketing fuel costs. The price of crude oil hit a record $100 a barrel in early January. It now hovers around $90.

United carried about 68 million passengers last year. Historically, about one in four passengers checks more than one bag, United says. It estimates this change will generate more than $100 million annually in fuel savings from fewer checked bags and revenue from the fee.

Discount giant Southwest Airlines last week implemented a $25 fee for checking a third bag.