- British Airways hit by online check-in failure.
- More than 60 flights from Heathrow and Gatwick were cancelled and more than 100 were delayed, according to the departure boards at the two airports.
- Airline urges customers to allow extra time.
Problems with British Airways' IT systems left thousands of passengers facing flight cancellations, delays and long queues at airports in the airline's third major computer failure in a little more than two years.
Wednesday's woes are the latest in a string of problems at the airline, which was fined $230 million last month for a huge customer data breach and is bracing for potential strikes in a pay dispute with its pilots.
BA, owned by International Airlines Group, apologized to customers for Wednesday's disruption and said its technical team was working to resolve the problems as soon as possible. It urged customers to allow extra time at airports.
More than 60 flights from Heathrow and Gatwick were cancelled and more than 100 were delayed, according to the departure boards at the two airports.
BA would not confirm how many people have been affected by the IT problems but said that it was experiencing a "systems issue" affecting check-in and flight departures at Heathrow, Gatwick and London City airports.
Customers have experienced issues checking in online, while others complained of being stuck on planes unable to take off for hours.
Stuart Jackson, a photography business manager, said he was stuck on a grounded plane at Heathrow and had already missed his connecting flight, disrupting months of planning and wasting thousands of pounds.
"When I do arrive, I will have to just fly home again," he said on Twitter. "BA is a complete catastrophe and I will never fly with them again."
A little more than a year ago BA was forced to cancel flights at Heathrow, Europe's biggest airport, after problems with a supplier's IT system. And in May 2017 a massive computer system failure because of a power supply issue left 75,000 customers stranded.
BA Chief Executive Alex Cruz vowed after that incident that the airline would take steps to ensure that computer system failures would never happen again.
British Airways said it has moved to back-up manual systems to keep some flights operating and it is offering customers the chance to rebook for another day.
Customers, meanwhile, continued to voice their anger.
Paul Trickett, a passenger at Heathrow, said his flight to Copenhagen was cancelled. He waited in line for 90 minutes to see an agent before an announcement told everyone to go home because no rebooking would take place at the airport.
Trickett had already been forced to rebook a flight from Tuesday after airport staff had threatened to go on strike.
"It's pretty chaotic," he told Reuters. "It would be quicker by ship."
Jamie Boswell said his flight was cancelled while he was attempting to check in. "Very busy to rebook. Not ideal with a one-year-old," he said.
Passengers trying to get Gatwick were also facing delays because of an earlier fire on railway tracks between Victoria and Clapham Junction stations, disrupting one of the main routes to the airport just outside London.