The delays, coming from air-traffic controller furloughs tied to $85 billion in across-the-board cuts known as the sequester, likely will affect some 6,700 flights a day starting Sunday, or nearly a third of the more than 23,000 daily U.S. flights, the Federal Aviation Administration said.
The agency told airlines the furloughs could delay twice as many flights as during last year's most heavily storm-disrupted days, people familiar with the briefing told The Wall Street Journal.
The briefing for airline executives came two days before Transportation Secretary Ray LaHood and FAA Administrator Michael Huerta briefed reporters.
LaHood told reporters Thursday he'd looked "far and wide" for ways of cutting $637 million from the FAA's $16 billion budget, as required by the sequester.
"But we cannot avoid furloughs. And these furloughs will begin Sunday," he said.
The department had to cut $1 billion from its overall budget to comply with the austerity fiscal policy that began March 1.
"This is very painful. This is not what we signed up for. This is a dumb idea. Sequester is a dumb idea. Not one person in America would use the sequester to figure out their budget. It's a meat-ax approach," LaHood said.
Congressional Republicans criticized the move, saying the department could have chosen less harmful cuts.
But LaHood, himself a Republican, said there was no way of eliminating $1 billion without cutting services.
He stressed the delays would not harm passenger safety.
"We're only going to handle the number of aircraft we can safely manage to fly," LaHood said. "I want to make it clear: Safety is not up for negotiations during sequestration."
The furloughs for the 55,000 Transportation Department employees -- including 47,000 FAA employees, most of whom are air-traffic controllers -- require them to take a day off without pay every 10 work days, or a total of 11 days off, through the end of the fiscal year ending Sept. 30.
Huerta said delays at Newark Liberty International Airport in New Jersey could average 20 minutes, with a maximum of 51 minutes, while Chicago O'Hare could see delays averaging 50 minutes, with a maximum delay of more than 2 hours.
The furloughs will lead O'Hare occasionally to shut down one of its two control towers, closing one runway and reducing takeoffs and landings, Huerta said.
Hartsfield-Jackson Atlanta, the nation's busiest airport, is expected to see delays averaging 11 minutes, but delays could be as much as 3 1/2 hours in a worst-case scenario, Huerta said.
Hartsfield safety limitations could prevent the FAA from using all five of the airport's runways, he said.
The maximum delays are "an estimate" and will probably be "infrequent," Huerta said, adding none of the estimates included unpredictable events such as equipment outages or weather.
Other airports the FAA forecast delays for are Kennedy International and LaGuardia in New York and Los Angeles International, Huerta said.
He said the FAA would soon release delay forecasts for Charlotte (N.C.) Douglas, Chicago Midway, Fort Lauderdale-Hollywood (Fla.), Miami, Philadelphia, San Diego and San Francisco.
Delays at all the big airports are likely to ripple to other airports, Huerta said.
Airlines for America, an industry group, said it was reviewing legal options to prevent the furloughs.
The National Air Traffic Controllers Association union said it would ask the White House to give the FAA the flexibility to avoid furloughing.
The Obama administration says sequestration, part of the Budget Control Act of 2011, permits no discretion with the budget cuts.
Sen. Jerry Moran, R-Kan., said: "The administration's insistence on deliberately inconveniencing air travelers instead of prioritizing their safety falls in line with its history of putting politics before common sense. ... They do have flexibility when it comes to sequestration."
Sen. Jay Rockefeller, D-W.Va., blamed Republicans for "refusing to work" for a budget compromise and criticized the FAA for not doing more to warn the public.
Airlines themselves did little so far to notify passengers of possible delays, in part because they hoped the FAA would alter its plans, executives told the Journal.
A United Press International check of the major U.S. airlines' websites Thursday night showed no notices about possible delays.
Most airlines declined to comment.
Southwest Airlines Co. told the Journal it would communicate with its customers about the delays "just as we do today during irregular operations."