More than 1,000 flights to and from the UK were grounded due to a “technical issue” on Tuesday afternoon, forcing tens of thousands of people to abandon their travel plans. Around 1,300 flights to and from UK airports were cancelled, according to Flightradar24, with a further 279 cancellations recorded early on Wednesday at Heathrow and Gatwick.About four hours after the disruption began, the National Air Traffic Control Service (Nats) said engineers had identified the problem and had fixed it.But airports have warned of further disruption as flights continue to be cancelled on Wednesday morning. Here, the Independent breaks down what caused the delays and how long the disruption is expected to last.Around 1,300 flights to and from UK airports were cancelled on Tuesday (PA)Why are there flight delays?The widespread disruption is due to a major technical failure within the UK's National Air Traffic Services (NATS) flight processing system, meaning it was not safe for planes to depart.The breakdown started at about 1pm on Tuesday and by the time Nats said it had fixed its flight processing system about four hours later, flights going to and from the UK were severely delayed. After another three hours, air-traffic chiefs admitted the recovery had taken longer than they had hoped – and the backlog of flights was expected to continue causing disruption into Wednesday.Which airports have been affected by the delays?Around 1,300 flights to and from UK airports were cancelled on Tuesday, according to Flightradar24, with a further 279 cancellations recorded early on Wednesday, at Heathrow and Gatwick.The following airports were affected: Heathrow Gatwick London City London Southend London Stansted Edinburgh Bristol Aberdeen Glasgow Southampton Jersey Manchester East Midlands Birmingham Belfast City Liverpool John Lennon The widespread disruption is due to a major technical failure within the UK's National Air Traffic Services (NATS) flight processing system (PA)Some airports are now resuming flights and passengers flying from Stansted airport today have been told to continue with their journey plans. This morning, the airport said that following yesterday’s Nats air traffic control issue that caused widespread flight disruption, Stansted is “operating as normal”.Manchester airport has also said flights are operating “as normal” with some delays to departures. Gatwick Airport is now operating flights, but some delays are still expected. The airport said that flights have now resumed, but “we still anticipate that it will take time for our operations to fully return to normal”.Heathrow Airport is also resuming operations after yesterday's air traffic control glitch. The airport said flights would be operating but "some disruption may continue as airlines reposition aircraft and crew".Passengers have been advised to check their flight status with their airline before travelling to the airport and only travel if their flight is confirmed to be operating.How long are the delays expected to go on for? It’s unclear how long the disruption will last at each airport, but the delays and cancellations have already spilled over into Wednesday morning.It will likely take hours to clear the backlog of flights with further knock-on disruption potentially lasting days.What caused the air traffic control problem? The chief executive of air traffic control provider Nats, Martin Rolfe, said the disruption was caused by an issue with Nats’ flight data processing system, adding that it was different from the system that failed during the August Bank Holiday weekend in 2023.He is due to speak with transport secretary Heidi Alexander as the disruption continues and is expected to tell MPs that a cyber attack was "categorically" not the cause of the fault.The cause of the fault is currently under investigation.