Chicago ? United Airlines, unable to stanch more than two years of swelling losses and spurned in a plea for federal assistance, will file today for federal bankruptcy court protection, a source close to the company said Sunday.

The Chapter 11 filing – five days after a federal panel declined to help the world’s No. 2 carrier cover its mounting debt – would be the largest airline bankruptcy in U.S. history, and one of the nation’s top 10 bankruptcies ever.

United operates about 1,700 flights a day, or about 20 percent of all U.S. flights. It plans to continue flying under bankruptcy protection. A later restructuring under a bankruptcy court would likely result in fewer flights, thousands of additional layoffs and other extensive cost-slashing.

A federal judge would be put in charge of overseeing the tangled financial affairs of United, which owes nearly $1 billion in overdue debt payments that would use up most of its remaining cash.

A spokesman for United’s pilot union urged passengers Sunday not to abandon the airline in the event it files for bankruptcy.

“We’re going to be flying airplanes, today, tomorrow, next week and next year,” pilots spokesman Herb Hunter said. “We don’t want people to give up on us, because we’re going to come through this.

“This is going to be painful for the stockholders and the employees, but the airline’s going to keep flying, and we’re going to come out of this stronger. The passengers shouldn’t notice any difference.”

Frequent-flier miles and basic fare levels are likely to be retained for the short term, although fare increases are likely over the longer haul.