CHICAGO — A computer problem this morning temporarily disrupted United Airlines flights at O'Hare International Airport, causing long delays and lines for travelers headed out for the Fourth of July holiday weekend.

The outage affected all of the United's computers at the airport and also caused some flight cancellations, said airline spokeswoman Robin Urbanski.

The problem started shortly after 5 a.m. and at one point, the airline instituted a "ground stop," keeping United flights bound for O'Hare grounded if they hadn't taken off yet, said Federal Aviation Administration spokeswoman Elizabeth Isham Cory.

She said the glitch affected the boarding pass system and the computers that tell pilots the number of passengers on board and weight of the plane, which affect how much fuel an aircraft can carry.

"You need that to take off," Isham Cory said.

The ground stop order was lifted shortly before 8 a.m., when the airline began resolving its computer issues, she said.

By 10 a.m., at least some of the computers used to check in passengers were working.

Because the outage involved United's ticketing system, the airline urged customers to monitor their flight status and check-in online before they got to O'Hare, Urbanski said. United employees manually processed customers checking in, and self check-in kiosks were closed, she said.