Chicago ? United Airlines, the nation’s second-largest carrier, warned Wednesday that it will file for bankruptcy protection this fall unless it succeeds in “dramatically” cutting costs.

The announcement steps up the pressure on unions that so far have balked at pay cuts United says are necessary to stem massive losses. The warning comes three days after US Airways filed for Chapter 11 bankruptcy protection and a day after American Airlines announced severe cutbacks.

“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,” Jack Creighton, chairman and chief executive officer of United parent UAL Corp., said in a statement.

United hopes the restructuring will help it win a $1.8 billion loan guarantee from the federal government, a prospect many industry watchers say is unlikely.

UAL, which has $2 billion in cash but is losing millions of dollars each day, must repay $875 million in loans due this fall.

United, the nation’s No. 2 carrier behind American, is setting a 30-day limit on cost-cutting talks with employees, vendors and leaseholders. Its pilots’ union tentatively agreed to a 10 percent pay reduction earlier this summer, but other groups have rejected proposed pay cuts.

Talks with employees, who own 55 percent of the company, are expected to focus on layoffs as well as pay cuts. United did not immediately follow the lead of American Airlines, which cut 7,000 jobs Tuesday and said it would retire planes and overhaul its flight schedule.

In addition to US Airways, Midway and Vanguard also filed for bankruptcy protection this year, citing the sluggish economy, competition from low-cost carriers and a downturn in travel after Sept. 11. United would be the biggest to seek protection from creditors.

Creighton said that the Elk Grove Village, Ill.-based airline would in the coming days present new cost-saving proposals to employee representatives and other stakeholders.

UAL’s stock fell 29 cents Wednesday to close at a 22-year low of $2.45 a share. It has lost 53 percent of its value this week.