The airline said it is suffering a “major IT systems failure” around the world. It didn’t say what was causing the problem but said there was no evidence of a cyber-attack.

A union official accused BA of cutting costs by laying off IT staff last year and outsourcing the work to India.

“This could have all been avoided,” said Mick Rix, national officer for aviation at the GMB union. “We can only feel genuinely sorry for the tens of thousands of passengers who are stranded at airports and face having their travel plans and holidays ruined.”

BA operates hundreds of flights from the two London airports on a typical day — and both are major hubs for worldwide travel.

Several hours after problems began cropping up Saturday morning, BA suspended flights up to 6pm (1700GMT) because the two airports had become severely congested. The airline later scrapped flights from Heathrow and Gatwick for the rest of the day.

Passengers at Heathrow reported long lines at check-in counters and the failure of the airline’s website and its mobile app. BA said the crash also affected call centers.

The airline said it is working to restore services out of Heathrow and Gatwick beginning Sunday, although there will still be some disruptions. It said it expected that London-bound long-haul flights would land on schedule Sunday.

One person posted a picture on Twitter of BA staff writing gate numbers on a white board.