British Airways has cancelled all flights from Gatwick and Heathrow as computer problems cause disruption worldwide.
Passengers have been told not to travel to the London airports because of "extreme congestion" at the terminals, with all BA planes grounded before 6pm on Saturday.
A spokeswoman for the airline said: "We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide."
BA added: "We've found no evidence that it's a cyber attack."
"The terminals at Heathrow and Gatwick have become extremely congested and we have cancelled all flights from Heathrow and Gatwick before 6pm UK time today, so please do not come to the airports," the airline continued.
"We are extremely sorry for the inconvenience this is causing our customers and we are working to resolve the situation as quickly as possible."
Travellers have been told to check the airline website and twitter account for updates about the situation.
It comes as scores of Britons headed overseas for the long weekend and half-term school holidays on Saturday morning.
The airline has experienced issues with its online check-in systems in the past.
Passengers were hit by severe delays in September and July last year because of IT glitches.
The latest problem meant parts of BA's website were unavailable and some travellers were unable check in on the mobile app, with many venting their frustration on social media.
Melissa Davis, who runs a legal PR agency in London, was held for more than an hour and a half on the tarmac at Heathrow, on a BA flight returning from Belfast.
Speaking from the plane, Miss Davis said the air conditioning had been off "so I don't think we will be going anywhere any time soon", but added that the passengers had been kept informed by their pilot and given water while they remained seated.
She later said she and others were then told they could not transfer to other flights because "they can't bring up our details".
Henry Tail, a 27-year-old teacher from London, claimed he had missed his flight to Rome because of the technical problems.
"I checked in online using the BA app at 8.15 for my flight at 9.25, then went and had breakfast," he told the Press Association.
"At some point, the app restarted and when I went to go through security, I couldn't log in to my booking to get my QR code.
"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 said.
Gareth Wharton, also at Heathrow Terminal 5, tweeted a picture of BA staff writing gate information on a whiteboard amid the systems outage.
"Gets worse, #T5 staff having to put gate info up on a white board #LowFi #Heathrow," he tweeted.
Passengers at Gatwick Airport faced chaotic scenes and long queues due to a baggage system problem on Friday.
Those taking flights were forced to travel without their hold bags and were asked to carry any essential items in their hand luggage.
Heathrow said it has posted extra staff at the terminals and warned BA passengers on flights before 6pm not to go to the airport.
"Passengers travelling with British Airways after 6pm should check the status of their flight before travelling to the airport," it added in a statement.
Dan Power said he and his 80-year-old grandmother are stranded at Heathrow, waiting for a flight to Milan, and have had no information from BA.
He told BBC News they have been offered "absolutely nothing" in the way of refreshments and added: "We haven't been offered any chairs, any water, any vouchers - nothing.
"I don't think our week's holiday will happen at this rate.
"My main concern now is I don't want my 80-year-old grandma spending the night on Heathrow floor - but all the hotels are fully booked, we don't have any transport back up north, so we are actually stuck in Heathrow with nowhere to go."
Several passengers at Heathrow told the Press Association they had not been informed their flights had been cancelled until more than an hour after the airline put out a press statement announcing the decision.
Images posted to social media showed a group of people gathered around the customer services desk at Heathrow's Terminal 5 trying to get information.
Shortly after the statement was released, Terry Page, 28, from London, said: "There's no such announcement here. The boards are showing 'Go to gate' and no mention of cancellations."
More than an hour later, he said cancellations of individual flights were still being announced.
"They've announced them 30 minutes apart - I think to prevent panic and mass exit," he suggested.
Footage filmed at Terminal 5 showed long queues at customer services, after passengers had been advised that they would be unable to rebook due to systems remaining down.
Student Emily Wilson told the Press Association that she had been advised "we are unable to get bags, and that no more flights are taking off", several hours after having arrived at the airport for her flight to Stockholm.
Ms Wilson added: "We were told (it would be) about three hours for collecting bags, that all compensation will have to be done online, and that we are unable to rebook flights now because of the system being down."
She said that information on screens still suggested her flight could board shortly, but that staff contradicted that information, saying there were "no slots left".