LONDON - British Airways says it is canceling all of its flights from London's Heathrow and Gatwick airports for the rest of the day because of a "major IT system failure."

BA has not said what is causing the computer problem, but says it has no evidence of a cyberattack.

The airline had earlier suspended flights up to 6 p.m. (1700 GMT) because the two airports had become severely congested.

The global computer failure has caused misery for tens of thousands of travelers on a key holiday weekend in Britain. Both airports - major hubs for worldwide travel - are overflowing with stranded, frustrated passengers.

Passengers at Heathrow reported long lines at check-in counters and the failure of the airline's website and mobile app. BA says the problems have also affected call centers.

BA says it's working to restore services beginning Sunday, although there will still be some disruption. It says it expects London-bound flights to land on schedule on Sunday.