Britain's National Air Traffic Service (NATS) was hit by a technical problem for several hours on Monday, causing widespread disruption to flights in UK airspace that it said would continue for some time even though the issue was fixed.

The air traffic control agency earlier had to restrict the flow of aircraft when its automatic processing of flight plans malfunctioned, requiring them to be handled manually and causing flight delays and cancelations.

Aviation analytics firm Cirium reported that on Monday, 790 flights scheduled to depart UK airports and 785 scheduled to arrive were canceled – in each case, equivalent to around 27 percent of the day's scheduled total. 

"It will take some time for flights to return to normal, and we will continue to work with the airlines and the airports to recover the situation," said NATS Operations Director Juliet Kennedy. "Our absolute priority is safety and we will be investigating very thoroughly what happened."

UK Transport Minister Mark Harper said he was working with NATS to help it manage affected flights and support passengers.

Irish air traffic control provider AirNav Ireland earlier said the issue, which struck during a public holiday in parts of Britain, was resulting in "significant delays for flights across Europe that are traveling to, from or through UK airspace."

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A spokesperson for London Heathrow, the busiest hub in western Europe, said schedules would remain significantly disrupted for the rest of the day.

"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 minimize the knock-on impacts and assist those whose journeys have been affected," the spokesperson said.

Cirium reported that by 8 a.m. GMT on Tuesday, 147 flights departing the UK and 134 arriving had been canceled – in each case, around 5 percent of the day's expected total, with the strong possibility of more to come. 

British Airways said its flights were severely disrupted and it had made "significant changes" to its schedule, while other airlines, including Ryanair, said some flights to and from the UK would be delayed or canceled.

Manchester Airport, London Stansted and London Gatwick were among the many UK airports that warned of delays and cancelations, while Dublin Airport said the problem affected some flights into and out of the Irish capital.

Many passengers earlier took to social media to say they were stuck on planes on the tarmac waiting to take off, or being held in airport buildings in Spain, Portugal, Greece, Israel and elsewhere on what is a traditionally busy travel day as the school holidays draw to a close.

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