NEW YORK (CNN/Money) - United Airlines parent UAL Corp. said Wednesday it may be forced to file for bankruptcy court protection in the fourth quarter due to pending debt payments.

Shares of UAL (UAL: Research, Estimates) plunged 60 cents, or nearly 25 percent, to $1.85 in after-hours trading on Instinet, after closing regular trading Wednesday before the announcement down 29 cents, or 10.5 percent, at $2.45.

The company said it has $875 million in debt payments coming due in the fourth quarter and insufficient access to capital markets to cover the payments. The company has asked for federal loan guarantees from the Air Transportation Stabilization Board, but company officials confirmed Monday that they were getting negative reaction to their application.

"Demand isn't returning, fares remain low, and the industry is grappling with how to respond," said a statement from interim CEO Jack Creighton. "At United, we have determined that we must make improvements in our business plan to ensure we get the cost savings we need to compete in an industry that has fundamentally changed. And our conclusions are consistent with the feedback we're getting from Washington."

The announcement comes three days after US Airways, the nation's No. 7 airline, filed for bankruptcy protection. US Airways is also seeking federal loan guarantees, but has won a conditional approval of its request, contingent on winning employees concessions.

United, the world's No. 2 airline after American Airlines, also announced a series of cost-cutting plans to deal with a tougher operating environment, and said it would hold negotiations with employees and suppliers to try to win more cost savings. But it said it has only 30 days to win the concessions it needs.

"Unless we lower our costs dramatically, filing for bankruptcy protection will be the only way we can ensure the company's future and the continued operation of our airline," said Creighton's statement.

While the Air Line Pilots Association leadership has given the company a tentative agreement on wage concessions, its two other major unions - the International Association of Machinists and the Association of Flight Attendants, have thus far refused to grant concessions.

The airline's statement said it will need to furlough more employees. At the end of June it had the equivalent of 79,800 full-time employees, down 18 percent from pre-terrorist attack levels of a year ago. A cut in its scheduled flights after the attack led to a 16 percent cut in capacity in the second quarter compared with a year earlier.

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United said it will need to make further adjustments to its schedule to improve network efficiency and manage air traffic. American Airlines announced such plans Tuesday, saying it would cut capacity by 9 percent in November and cut 7,000 employees, or about 6 percent of its staff, by next March as a part of the restructuring of the business. United did not give any details of its cost-cutting plans.

Two airline analysts have predicted possible bankruptcy filings for the carrier within the last week. While the company reported unencumbered cash balance of $2.4 billion at the end of the second quarter, it warned in its second-quarter financial report last month that its cash burn rate of less than $1 million a day in the second quarter was expected to rise in the third and fourth quarters.

Lehman Brothers analyst Gary Chase estimated there would be a $4 million cash burn rate for UAL, and Jim Higgins of Credit Suisse First Boston estimated that the company would have less than $300 million left by the end of the year.

"An airline needs a significant amount of cash to operate," he said. "You don't run it to zero."

The company needed to fill 87 percent of its seats to break even in the second quarter, and only filled 74.4 percent of the seats.