Airline passengers have been warned that flight disruption could persist for days, after a technical meltdown in UK air traffic control left hundreds of thousands of passengers stranded or delayed on the summer bank holiday. Returning holidaymakers and those hoping to travel out of UK airports faced cancellations and delays of up to 12 hours after takeoffs and inbound flights were suspended due to a “network-wide” computer failure. A limited number of flights were able to operate but air traffic was severely restricted as engineers struggled to locate and rectify the problem. With controllers forced to input flight plans manually, about 500 flights were cancelled and others delayed for hours even before Nats, the national airspace controllers, announced at 3.15pm that it had “identified and remedied” the issue that arose almost four hours earlier. Passengers at airports in the UK and around Europe reported being left in limbo with travel plans wrecked and check-in desks closed, while airlines were unable to confirm if their flights would leave. The unusually long outage is likely to cause disruption for several days, with knock-on delays from crew and planes left out of position. British Airways said passengers due to travel on Monday or Tuesday could move their flights free of charge, while Heathrow on Monday evening urged passengers to come to the airport only if flights were confirmed as operating. At Gatwick, where about 150 flights were scrapped, easyJet cancelled virtually all departing international flights on Monday afternoon. The airline could not yet confirm what flights would operate on Tuesday but it is understood to expect some continuing impact on its schedules. According to flight tracking sites, planes were delayed from about 11.30am. Nats confirmed the problem at about 12.10pm, saying it was “currently experiencing a technical issue” and had “applied traffic flow restrictions to maintain safety”. After announcing it had fixed the original issue, Nats said: “We are now working closely with airlines and airports to manage the flights affected as efficiently as possible. Our engineers will be carefully monitoring the system’s performance as we return to normal operations. “Our priority is always to ensure that every flight in the UK remains safe and we are sincerely sorry for the disruption this is causing. Please contact your airline for information on how this may affect your flight.” According to data from the analytics firm Cirium, 232 outbound flights from the UK and 271 inbound flights had been cancelled by 2.30pm, just under 10% of all services. A Heathrow spokesperson said schedules would “remain significantly disrupted for the rest of the day”. They added: “We ask passengers to only travel to the airport if their flight is confirmed as still operating. Teams across Heathrow are working as hard as they can to minimise the knock-on impacts and assist those whose journeys have been affected.” A British Airways spokesperson said: “Like all airlines using UK airspace, our flights have been severely disrupted … While Nats has now resolved the issue, it has created significant and unavoidable delays and cancellations. We’re working as hard as possible to get customers whose flights have been affected on their way again and have apologised for the huge inconvenience caused.” The travel operator Tui warned its customers to expect “significant delays to some of our flights”. Flights from Ireland were also affected, with many due to cross UK airspace. As well as holidaymakers, passengers affected by delays included British athletes and others returning from the World Athletics Championships in Hungary. The BBC presenter Gabby Logan said on X that her plane was stuck on the runway at Budapest airport, adding: “After almost three weeks away from home I am hours from hugging my family. And have just been told UK airspace is shut. We could be here for 12 hours. So we sit on the plane and wait.” Engineers at Nats will have been racing against the clock to limit the fallout from the outage, potentially the most serious since its control centre in Swanwick, Hampshire, opened in 2002, falling on one of the key travel dates in the calendar. A computer glitch at Nats in 2014 affected flights until the following day, despite airspace being curtailed only for about an hour. Labour and the Liberal Democrats questioned the government’s apparent slow response before the transport secretary, Mark Harper, posted on X on Monday afternoon that ministers were “doing all we can”. Harper said: “UK airspace remains open but traffic flow restrictions are in place. Nats are working at pace to fix this and aviation minister [Charlotte Vere] and I are doing all we can to support them.” Earlier, the shadow transport secretary, Louise Haigh, posted that the issue was “extremely concerning for passengers travelling in and out of the UK on one of the busiest days of the year”, adding that she was “surprised” by the lack of a ministerial statement. The Lib Dems called on Rishi Sunak to convene a Cobra meeting. The party’s transport spokesperson, Wera Hobhouse, said: “Millions of holidaymakers could be facing huge disruption in the coming days due to this fault and we can’t risk this government being missing in action yet again.”