ATLANTA — Delta Air Lines Inc., the nation's third-largest carrier, will double its charge for checking a second bag on a domestic flight as part of a set of fee increases to help offset the high cost of fuel.

The Atlanta-based carrier said yesterday that the changes will apply to people who purchase a ticket tomorrow or after, for travel on or after Tuesday.

Fee changes include an increase from $25 to $50 to check a second bag for domestic travel. Fees for specialty items that require special handling, such as surfboards or ski equipment, will increase on domestic and international flights, Delta said.

First Class, BusinessElite and Medallion customers will still be able to check up to three bags at no charge, the airline said, and international travelers will be able to check two bags without charge.

Delta said that as fuel costs remain high, it believes revising its fee structure for excess bags and specialty items is essential to generate the necessary revenues to offset its higher costs.

Delta does not charge passengers for checking a first bag on domestic flights. It has said it is studying a decision by several other major carriers to impose that fee.

Fees are for each extra bag, each way.

"They're trying to have the second-bag charge subsidize keeping the first bag free," said Jay Sorensen, who runs Ideaworks, an aviation consulting firm in Shorewood, Wis.

The Delta fee tops the $25 second-bag charge at AMR Corp.'s American, UAL Corp.'s United, Continental Airlines Inc. and Northwest Airlines Corp., which round out the five biggest full-fare carriers. Southwest Airlines Co., the largest discounter and No. 5 overall by traffic, doesn't have fees for the first two bags.

United and Northwest are among the carriers that have joined American in charging $15 for the first checked bag.

Fewer than 20 percent of all Delta passengers check a second bag, spokeswoman Betsy Talton said. She declined to say how much additional revenue the new fees will generate.

Also, Delta said the fee for checking a third bag on a domestic flight will increase from $80 to $125.

Delta warned on its Web site that if a customer has an extra piece of baggage that goes over the weight and size limits, the passenger will be charged three times — once for the extra bag, once for going over the size limit and once for going over the weight limit.

The airline said it accepts up to 10 checked bags on Delta mainline flights and four checked bags on Delta Connection carriers.

Delta announced April 14 that it will acquire Eagan, Minn.-based Northwest Airlines Corp. in a stock-swap deal that would create the world's largest carrier in terms of traffic. The deal is subject to shareholder and regulatory approval.

Bloomberg News Service contributed to this report.