Federal transportation officials told the nation's airlines at the start of negotiations yesterday that they would impose schedule changes at Chicago's congested O'Hare International Airport if the carriers didn't cut flights voluntarily.

In the past 10 months, nearly one-third of all arrivals at O'Hare have been delayed and one in three delayed flights is late by more than an hour. Even the slightest hiccup at O'Hare tends to ripple through the nation's air travel system because half of the airport's passengers are connecting to other destinations.

"What we have before us is an urgent and unusual and important piece of business," Transportation Secretary Norman Y. Mineta said, kicking off what will likely be two days of intense negotiations with 15 carriers that operate at the hub. "It's not a meeting any of us really wanted to have. But the conclusion is inescapable. O'Hare and their on-time performance is unacceptable."

If airlines do not cut back enough, FAA Administrator Marion C. Blakey said her agency would do it for them. "We will take action unilaterally if we can't come to an agreement," Blakey told airline representatives before the meetings began. "If we need to, we'll issue an order requiring the most efficient schedule that reflects our best judgment, which may not be yours."

The FAA is asking each carrier to voluntarily eliminate or move flights at O'Hare between 7 a.m. and 9 p.m. beginning Nov. 1.

Under a law passed in 2003, Congress gave the Transportation Department authority to restrict schedules at severely congested airports, marking the first time since government deregulation of the industry in 1978 that the government can begin managing flight schedules at select locations. Before the authority is used, the agency first must work with individual airlines to encourage them to make the cuts voluntarily.

A surge in airline traffic this summer has hit O'Hare harder than other airports. O'Hare serves as a hub for the nation's two largest carriers, American and United, which have added more flights using smaller planes to improve efficiency. American also shifted more flights to Chicago from its hub in St. Louis. In addition, low-fare carriers such as Independence Air have jumped in to stimulate competition.

On many days at O'Hare, the result is that during certain hours, too many airlines add more flights than can be safely scheduled for takeoff and landing. If a summer thunderstorm sweeps through the area, the backlog of flights grows dramatically. As a result, passengers are commonly delayed or miss connecting flights. The airport commonly sets up cots for stranded passengers to sleep at the terminal.

Before the meeting, some carriers complained that cutting back scheduled flights would hurt business. Independence Air, a new low-fare carrier based at Dulles International Airport, said it was worried that further cuts to its schedule would reduce competition with major carriers such as American and United, which have 88 percent of the flights at O'Hare.

Independence's counsel, Robert Silverberg, and senior director of market planning, Jeff Pollack, met privately with FAA officials for less than an hour yesterday afternoon. Rick DeLisi, the airline's spokesman, declined to comment on whether Independence offered to reduce any of its service to O'Hare. He pointed out that Independence operates just 12 of the 1,370 flights that arrive at O'Hare every day.

Other carriers, such as United, said they were willing to participate but wanted the cuts to be evenly spread.

"We'd expect any short-term proposal would need to be equitable among all the airlines," said United spokeswoman Jean Medina.

In the end, airlines are likely to go along with cuts rather than have the government impose them, said Darryl Jenkins, director of the Aviation Institute at George Washington University. "If the FAA wanted to, they could do all sorts of things in and out of [O'Hare] that would be extremely odious," Jenkins said. "Everyone will feel pain. The question is, how much pain are you willing to accept? If you're in on the negotiations, you can reduce the amount of pain you're going to suffer."

Staff writer Bill Brubaker contributed to this report.