Media playback is unsupported on your device Media caption Queues at Heathrow airport stretched out of the terminal 
 
 Serious problems with British Airways' IT systems have led to thousands of passengers having their plans disrupted, after all flights from Heathrow and Gatwick were cancelled. 
 
 Passengers described "chaotic" scenes at the airports, with some criticising BA for a lack of information. 
 
 The airline has apologised, and told passengers not to come to the airport. 
 
 BA chief executive Alex Cruz said: "We believe the root cause was a power supply issue." 
 
 In a video statement released via Twitter, he added: "I am really sorry we don't have better news as yet, but I can assure you our teams are working as hard as they can to resolve these issues." 
 
 Mr Cruz said there was no evidence the computer problems were the result of a cyber attack. 
 
 The airline hoped to be able to operate some long haul inbound flights on Saturday, landing in London on Sunday, Mr Cruz added. 
 
 The GMB union has suggested the failure could have been avoided, had the airline not outsourced its IT work. 
 
 BA denied the claim, saying: "We would never compromise the integrity and security of our IT systems". 
 
 All passengers affected by the failure - which coincides with the first weekend of the half-term holiday for many in the UK - will be offered the option of rescheduling or a refund. 
 
 The airline, which had previously said flights would be cancelled until 18:00 BST, has now cancelled all flights for Saturday and asked passengers not to come to Gatwick or Heathrow airports. 
 
 Other airlines flying in and out of the two airports are unaffected. 
 
 Architect and TV presenter George Clarke was stuck in Heathrow. He told the BBC it was one of the "most turbulent, badly organised days, that I've ever experienced in Britain". 
 
 "The lack of communication all day was woeful. There wasn't a single Tannoy announcement all day in the terminal, not a single member of staff came up to us," he said. 
 
 "The only time I found out my flight was cancelled was from the BBC News website." 
 
 Image copyright @AnnaOnTheWeb Image caption Piles of checked luggage could be seen on the floor in the Heathrow 
 
 Image caption Some passengers have reported having to leave Heathrow without their luggage 
 
 The problems have affected BA call centres, the website and the mobile app. 
 
 Aviation expert Julian Bray said: "It's frozen the whole system so no British Airways plane can actually take off, they can't move the baggage, they can't issue passenger credentials, in fact they can't do anything at all. 
 
 "This is a very serious problem, they should have been able to switch to an alternative system - surely British Airways should be able to do this." 
 
 Malcolm Ginsberg, editor in chief at Business Travel News, expects the disruption to last for "three or four days". 
 
 BA aircraft landing at Heathrow are unable to park as outbound aircraft cannot vacate the gates, which has resulted in passengers being stuck on aircraft. 
 
 Journalist Martyn Kent said he had been sitting on a plane at Heathrow for 90 minutes. He said the captain told passengers the IT problems were "catastrophic". 
 
 'Catastrophic' 
 
 Mick Rix, GMB's national officer for aviation said: "This could have all been avoided. 
 
 "BA in 2016 made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India... many viewed the company's actions as just plain greedy." 
 
 BA staff in Heathrow's Terminal 5 were resorting to using white boards, according to passenger Gareth Wharton. 
 
 Delays have been reported in Rome, Prague, Milan, Stockholm and Malaga due to the system failure. 
 
 Philip Bloom said he had been waiting on board a Heathrow-bound flight at Belfast for two hours. 
 
 He added: "We haven't been told very much just that there is a worldwide computer system failure. 
 
 "We were told that we couldn't even get on other flights because they are unable to see what flights we can be moved to." 
 
 Analysis - By Richard Westcott, BBC transport correspondent 
 
 Image copyright @theboyg Image caption With a lack of technology, staff were using whiteboards in Heathrow 
 
 As ever, it is a lack of information that is really making BA passengers angry… we're still awaiting an explanation from the airline and a timescale for how long the problems might last. 
 
 The GMB union says this meltdown could have been avoided if BA had not made hundreds of IT staff redundant and outsourced their jobs to India at the end of last year. 
 
 Yes, the union has a big axe to grind, but people will want to know if the airline made its IT systems more vulnerable by scaling back computer support to save money - although BA has just flatly denied it to me. 
 
 IT problems ripple through an airline. If planes cannot take off, they cannot leave gaps at the gate for others to land. 
 
 If flights are delayed by more than around five hours, the airline must swap crews because shift lengths are strictly limited for safety reasons. 
 
 Telling customers to stay away is a drastic measure, but it is the only chance BA has of clearing the backlog of flights. 
 
 Image copyright Twitter 
 
 The BBC's Phillip Norton was at Rome international airport, waiting to fly to London. 
 
 He said BA staff were unable to say how long delays would be, telling him "all flights are grounded around the world". 
 
 Alma Saffari was in Marseille waiting to get her flight back to Heathrow. 
 
 She said: "When we finally boarded the captain came out and told us their computer systems were down worldwide. 
 
 "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." 
 
 Ms Saffari, who is with her 13-month-old baby, said she had been given a voucher for food and drink. 
 
 EU flight delay rights 
 
 Image copyright @TimReidCE Image caption Passengers have experienced large queues and disruption at Heathrow Terminal 5, British Airways' main London terminal ||||| London (CNN) British Airways canceled all Saturday flights from London's two biggest airports after "a major IT system failure" caused severe disruption to flight operations worldwide, the airline said. 
 
 "Our IT teams are working tirelessly to fix the problems," British Airways CEO Alex Cruz said in a video posted on Twitter. 
 
 "We believe the root cause was a power supply issue and we have no evidence of any cyberattack." 
 
 The airline initially canceled only flights before 1 p.m. ET (6 p.m. BST) at Heathrow and Gatwick. 
 
 "We are working hard to get our customers who were due to fly today onto the next available flights over the course of the rest of the weekend," the airline said in a statement Saturday afternoon. "Those unable to fly will be offered a full refund." 
 
 The system outage -- on Britain's spring bank holiday weekend -- has "affected our call centers and our website but we will update customers as soon as we are able to," the airline said. 
 
 "We are extremely sorry for the huge inconvenience this is causing our customers and we understand how frustrating this must be, especially for families hoping to get away on holiday," Cruz said. 
 
 The airline posted a statement on its website on Sunday saying "although some of the IT systems have returned, there will be some knock-on disruption to our schedules as aircraft and crews are out of position around the world." 
 
 "At this stage we are aiming to operate a near normal schedule of flights from Gatwick and the majority of our Heathrow services," British Airways posted. 
 
 British Airways has not outlined how many flights were canceled Saturday, nor how many people were affected. 
 
 "Most long-haul flights due to land in London tomorrow (Sunday, May 28) are expected to arrive as normal, and we are working to restore our services from tomorrow, although some delays and disruption may continue into Sunday," the statement said. 
 
 In response to tweets from travelers, the airline said it was experiencing a "global system outage" affecting its website, online check-in, contact centers and baggage tracing. 
 
 "Working on this as a matter of priority, but if your flight is departing tomorrow you may have to complete check in at the airport," it tweeted to one customer. 
 
 Passengers wait Saturday at London's Heathrow Airport as British Airways experiences computer problems. 
 
 Airline workers 'totally crippled' 
 
 Travelers in the United Kingdom were already reporting disruptions, including baggage and flight delays, when the airline reported serious computer problems around midday. 
 
 We're working with @British_Airways following an issue with their IT system this morning that has caused some delays for passengers. 1/2 — Heathrow Airport (@HeathrowAirport) May 27, 2017 
 
 Paddy Anigbo, a Nigerian national visiting his wife in the United Kingdom on their wedding anniversary, said his plane sat on the tarmac at Heathrow for about four hours after landing. The plane's entertainment system and air conditioning were operating, and free snacks and drinks were served. 
 
 But, he told CNN by phone: "No alcohol, which is wise. You don't want that kind of chaos." 
 
 "There's no real frustration," Anigbo, an IT specialist, said while still on the tarmac. "I mean, the frustrations are there. We all want to get off the plane and go home." 
 
 Anigbo praised the flight crew and lamented that the pilot, at one point, announced that he would give hourly updates. 
 
 "What's pretty clear is that the situation in the terminal is quite serious and the IT systems are being worked on and this is going to take a while," he said. 
 
 When Anigbo's plane finally reached the gate, he described the scene as "chaos." 
 
 "We don't even know where to pick up our luggage," he said. 
 
 Finally packed at the gate and coming off soon. https://t.co/wUF2BjsTe4 — Paddy Anigbo (@dirosky) May 27, 2017 
 
 Gareth Ebenezer, who was on his way to Dublin to watch a rugby union final, told CNN he gave up and went home after getting caught up in the chaos at Heathrow. 
 
 Ebenezer saw lots of slow-moving lines, with some people asked to wait outside Terminal 5, he said. Most travelers seemed resigned, he said, though others appeared stressed and some children were crying. 
 
 Most people were being rebooked on Sunday flights, he said, but by then, he'd already have missed his match. 
 
 "BA staff were slow on announcements and updates, and were bringing out cordons and bottles of water to manage queues," Ebenezer said. "They seemed incapable of doing anything else, despite their best intentions, due to the system outage. They were totally crippled." 
 
 'Long line of planes' 
 
 Some stranded passengers took to Twitter to voice their frustrations. 
 
 Stuck at Heathrow Terminal 5. No updates or staff around. Have been stuck in a plane for 1 hour, and now at the airport for 2 #ba #heathrow — Wild Way Round (@wildwayround) May 27, 2017 
 
 "Stuck at Heathrow Terminal 5. No updates or staff around," travel writer and blogger @WildWayRound tweeted. "Have been stuck in a plane for 1 hour, and now at the airport for 2." 
 
 Staff couldn't find her baggage, including the gear she needs for a marathon Sunday, she said. 
 
 Another non-update update. I have only seen 1 plane leave terminal 5 in the last hour. Long line of planes on the tarmac. #BA #Heathrow — Don Bowman (@GadgetGuyNC) May 27, 2017 
 
 "Long line of planes on the tarmac," tweeted Don Bowman of North Carolina, adding that he'd seen only one plane leave Terminal 5 in the previous hour. 
 
 Sadly stuck on tarmac at Heathrow for at least an hour. No space for us to park. It continues... #britishairways #heathrowdelays — Philip Bloom (@PhilipBloom) May 27, 2017 
 
 Filmmaker Philip Bloom tweeted that he was stuck on the tarmac at Heathrow after his delayed flight from Belfast, Northern Ireland, finally landed because there was no space for the aircraft to park. 
 
 9 hours after checking in at Belfast airport we get home after the chaos of the British Airways IT crash. I docume... https://t.co/aYNq2qVvlo pic.twitter.com/HZ52FRbSjl — Philip Bloom (@PhilipBloom) May 27, 2017 
 
 Bloom later tweeted, "9 hours after checking in at Belfast airport we get home after the chaos of the British Airways IT crash." 
 
 Previous airline outages 
 
 Computer glitches and outages have plagued BA and other airlines in recent months. 
 
 In September, British Airways said a computer problem in its check-in systems caused delays and long lines at numerous airports.
– British Airways canceled all flights out of London's two largest airports Saturday—the first day of a spring bank holiday weekend in Britain—affecting tens of thousands of travelers. The problem stems from British Airway's IT system, the entirety of which apparently crashed worldwide, the AP reports. According to CNN, the airline says it doesn't appear to have been a cyberattack, but rather a "power supply issue." An aviation expert tells the BBC that British Airways "can't do anything at all." That includes having planes take off, moving baggage, and issuing credentials to passengers. The problem affected call centers, the British Airways website, and its mobile app. While British Airways hopes to have some flights in the air Sunday, the problem could persist for days, one expert says. On a normal day, British Airways has hundreds of flights out of Heathrow and Gatwick. On Saturday, travelers described chaos at the airports. One such traveler at Heathrow called it one of "most turbulent, badly organized days that I've ever experienced in Britain." With planes unable to take off and blocking gates, passengers on flights arriving at the airports were unable to disembark. Some waited on their planes for four hours only to get off and learn they had no idea where their luggage was. British Airways eventually told passengers to stop coming to the airport. Those affected will be offered a rescheduled flight or refund.