LONDON — British Airways canceled all flights from London’s Heathrow and Gatwick airports on Saturday as a global IT failure upended the travel plans of tens of thousands of people on a busy U.K. holiday weekend.

The airline said it was suffering a “major IT systems failure” around the world. Chief executive Alex Cruz said “we believe the root cause was a power-supply issue and we have no evidence of any cyberattack.”

He said the crash had affected “all of our check-in and operational systems.”

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 6 p.m. because the two airports had become severely congested. The airline later scrapped flights from Heathrow and Gatwick for the rest of the day.

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

The problem comes on a bank holiday weekend, when tens of thousands of Britons and their families are traveling.

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

Passenger Phillip Norton tweeted video of an announcement from a pilot to passengers at Rome’s Fiumicino airport, saying the problem affects the system that regulates what passengers and baggage go on which aircraft.

The pilot said passengers on planes that have landed at Heathrow were unable to get off because there was nowhere to park.

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

“We’ve tried all of the self-check-in machines. None were working, apart from one,” said Terry Page, booked on a flight to Texas. “There was a huge queue for it and it later transpired that it didn’t actually work, but you didn’t discover that until you got to the front.”

Another traveler, PR executive Melissa Davis, said her BA plane was held for more than 90 minutes on the tarmac at Heathrow on a flight arriving from Belfast.

She said passengers had been told they could not transfer to other flights because “they can’t bring up our details.”

Some BA flights were still arriving at Heathrow on Saturday, although with delays.

American Airlines, which operates code-share flights with BA, said it was unaffected.