UNITED Airlines says it will honour the tickets it accidentally gave away for free.

The decision is good news for people who snapped up the tickets on Thursday after United listed airfares at $0. Many customers got tickets for US$5 ($5.40) or US$10, paying only the cost of the September 11 security fee.

The mistake was an especially good deal for any passengers who bought tickets for travel within the next week. For instance, a Houston-to-Washington Dulles flight for next weekend would have cost US$877, according to United's website on Friday.

Airlines have posted so-called mistake fares before. They generally decide on a case-by-case basis whether they'll honour the ticket. On Friday, United Continental Holdings Inc said it was honouring the tickets "based on these specific circumstances.'' On Thursday, United said there was an error in filing the fares to its computer system.

United would not say how many tickets it accidentally gave away, or how much the mistake cost. The wrong fares were available on its website for a few hours on Thursday. It eventually shut down bookings on the website until it could fix the problem.

Some passengers bought their tickets for flights on Friday, and then immediately checked in online and printed their boarding passes, figuring that would give them their best chance of being able to use their ticket.

Mike Nehmer bought a ticket for him and his wife to fly from Houston to San Francisco on Friday. But he chickened out after a United agent told him that his ticket would be rejected at the gate. He ended up changing his ticket to December.

"I decided it wasn't in my best interest to show up and be denied at the gate,'' he said. "I have a pretty unhappy wife, too.''

