British Airways said Saturday that it had cancelled all its flights out of major London airports Heathrow and Gatwick after an IT systems failure, warning people not to travel to the congested hubs.
"We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide," BA said in a statement.
"The terminals at Heathrow and Gatwick have become extremely congested and we have cancelled all flights from Heathrow and Gatwick before 6.00 pm UK time today (1700 GMT), so please do not come to the airports."
The airline said it had found "no evidence that it's a cyberattack," with Britain still recovering from a ransomware attack that crippled key infrastructure earlier this month.
Travellers wrote on Twitter that they were unable to check in or use the airline's app.
Henry Tail, a 27-year-old teacher, told the Press Association news agency that the glitch forced him to miss his flight.
"When I went to go through security I couldn't log in to my booking to get my code," he said.
"This meant I couldn't go through security, and by the time I'd gone back and forth to various customer service desks the flight had closed," he added.
Others said they had been left stranded on the tarmac.
"We are on the runway.. For hour now.. no offer of drinks.. and because ba only takes cards now, we only have cash...wat we supposed to do!!??," Julie Adie wrote on Twitter.
Another Twitter user called Jo called it a "terrorist's dream", writing that "people getting off plane with/without luggage. Some staying on."
Inside Heathrow's Terminal 5, Luke Hallard wrote that "everyone remarkably calm despite British Airways chaos, but wait until the lounge runs out of booze".
Delays were also reported in Rome, Prague, Milan, Stockholm and Malaga, with the airline unable to say when flights would resume during the busy bank holiday weekend.
Alma Saffari told the BBC that her flight from Marseille to London had been grounded.
"When we finally boarded the captain came out and told us their computer systems were down worldwide," she said.
"Eventually after sitting on the tarmac for one and a half hours we disembarked the plane.
"Now we are sitting in the departure area outside the gate."
The airline has suffered other IT glitches recently, leading to severe delays for passengers in July and September last year.