British Airways cancelled all flights from London’s Heathrow and Gatwick airports Saturday as a global IT failure causes severe disruption for travellers.

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

BA said terminals at Heathrow and Gatwick had become extremely congested and cancelled all flights from those airports until 1700GMT. It is urging passengers not to go to the airports.

BA has not said what is causing the computer failure. It said in a tweet that the problem is global.

Earlier, passengers at Heathrow reported long lines at check-in counters and flight delays.

One 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 traveller, PR executive Melissa Davis, said she was held for more than an hour and a half on the tarmac at Heathrow aboard a BA 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.”

The problem comes on a holiday weekend, when thousands of Britons are travelling.

BA passengers were hit with severe delays in July and September 2016 because of problems with the airline’s online check-in systems.