British Airways has cancelled all flights from London's Heathrow and Gatwick airports today because of a "major IT system failure".
The carrier earlier announced the cancellation of all flights before 6pm due to the failure which it said was causing very severe disruption to our flight operations worldwide".
The airline earlier today apologised to travellers for an "IT systems outage" which had led to passengers complaining of "huge queues" at check-in along with problems using the airline's website and app.
Passengers have been told not to travel to the London airports because of "extreme congestion" at the terminals.
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."
BA later cancelled all flights on Saturday.
Travellers have been told to check the airline website and twitter account for updates about the situation. Many expressed frustrations over the outage on social media.
The delays came 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.
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, 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.
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.