The two-hour outage held up a total of 636 of the 5,679 United flights scheduled for Thursday, the airline said. That included 257 planes delayed directly by the outage, with the rest caused by planes that were late to arrive for subsequent flights, the airline said.
From Los Angeles to London, Boston to San Francisco, frustrated fliers tweeted snarky remarks about the problem. It was United's third major computer mishap this year.
"Does anyone have a Radio Shack computer or abacus to help United get their system fixed?" tweeted Lewis Franck, a motorsports writer flying from Newark, N.J., to Miami to cover the last race of the NASCAR season.
In a subsequent phone call with The Associated Press, Franck added: "Why is there a total system failure on a beautiful day? What happened to the backup and the backup to backup?"
United said the technology problem occurred around 8:30 a.m. EST and was fixed by 10:30 a.m. But morning delays can ripple throughout an airline's network for the rest of the day even after the underlying cause is fixed. That's because once a plane departs late, it can be hard to make up for lost time.
The glitch involved communication between dispatchers at the company's operations center in Chicago and planes at airports around the world, United spokesman Rahsaan Johnson said. Dispatchers communicate information such as weight and fuel loads to pilots, who need it to operate the flight. Johnson said the airline has identified the specific problem, and said it won't happen again.
The stock price of United Continental Holdings Inc. fell 47 cents, or 2.4 percent, to $19.51 on a day when shares of other big airlines rose.
United has been struggling with technology problems since March, when it switched to a passenger information computer system that was previously used by Continental. United and Continental merged in 2010. That system, called "Shares," has needed extensive reworking since March to make it easier for workers to use.
In August, 580 United flights were delayed and its website was shut down for two hours because of a problem with a piece of computer hardware.
Johnson said the problems on Thursday were not related to integrating the computer systems of the two airlines.
He said 10 Thursday flights were canceled because of the problem. He said 80 percent of the airline's flights were still on time. By comparison, government statistics show United and Continental each with about 83 percent of flights on time in November 2011.
He said that the problem affected planes that came from United. Planes that came from Continental, and regional flights on United Express, were not affected.
CEO Jeff Smisek acknowledged on Oct. 25 that some customers avoided United over the summer because of its computer problems. He said the airline had fixed those problems by improving software and adding more spare planes to its system, among other moves.
"We expect to earn back those customers that took a detour and we expect to attract new customers as well," he said at the time.
Thursday's problems were exactly what United did not need, said airline and travel industry analyst Henry H. Harteveldt of Atmosphere Research Group. "This event shows an unacceptable lack of planning at United," he said.
"This merger has been an outright disaster on almost every count. United must make some changes in its executive leadership, starting with the CEO" and including its chief information officer if it wants to restore confidence among passengers, he said.
That confidence appeared shaken on Thursday.
Michael Silverstein, who works in finance, was supposed to be on a 6:01 a.m. flight from Los Angeles to San Francisco. The computer outage had already caused him to miss one meeting. Worried about missing another, he walked off the plane and bought a $195 last-second ticket on a Southwest Airlines flight to Oakland, Calif.
"I'm frustrated because I'm missing a meeting that I thought I had plenty of time for," he said.