United Continental Holdings Inc., bedeviled by computer failures since the carrier’s creation in a 2010 merger, experienced new disruptions with the system that handles check-ins and other passenger services.

Airport and online check-ins returned to normal shortly after noon New York time, about three hours after the incidents began, said Megan McCarthy, a spokeswoman for the second-largest U.S. airline. United hasn’t determined the cause, she said.

Travelers in San Francisco, Washington, Chicago and other cities used social media to complain of check-in delays at airports, online and via mobile applications and difficulty in rebooking missed flights. Chicago-based United answered via Twitter that it was having “intermittent systemwide issues.”

That “has resulted in some pretty long lines in their lobby for check-in,” Larry Mares, duty manager at San Francisco International Airport, a United hub, said in an interview. “There are some delays resulting from that.”

Computer woes have dogged the carrier as it meshes operations after the combination of former parent UAL Corp. and Continental Airlines Inc. A malfunction in January stranded pilots and caused about 1,500 flight cancellations, following failures in 2012 that delayed trips and gave pilots a faulty weight estimate of a plane before takeoff.

The passenger service system, known as Shares, suffered a failure of less than 30 minutes starting at 9 a.m. New York time today, followed by sporadic disruptions until just after midday, McCarthy said. She couldn’t immediately say how many flights were delayed.

“We’re currently working to get all our customers to their destinations as soon as possible,” she said.

The issue also affected airport kiosks, ticket readers at gates and baggage-tag printers, McCarthy said.

Editors: Ed Dufner and John Lear.

To contact the reporter on this story: Mary Schlangenstein in Dallas at maryc.s@bloomberg.net. To contact the editor responsible for this story: Ed Dufner at edufner@bloomberg.net.

Photo Credit: In this Saturday, Dec. 21, 2013, file photo, Holiday travelers check in at the United Airlines ticket counter at Terminal 1 in O'Hare International Airport in Chicago. United Airlines reports quarterly earnings on Thursday, Jan. 23, 2014. Nam Y. Huh / AP Photo