A technical failure that saw hundreds of flights delayed and canceled across the U.K. was caused by problems in some flight data received by Britain’s national air traffic controllers and was not a cyberattack, authorities said Tuesday.

The breakdown, which hit on a late-summer holiday Monday that is one of the busiest days of the year for air travel, saw thousands of travellers stranded at airports. Transport Secretary Mark Harper said it was the worst incident of its kind in almost a decade and it would take days to “get people back to where they should be.”

The National Air Traffic Services said initial investigations showed that the problem “relates to some of the flight data we received.” It gave no specific information on what caused the trouble.

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“Our systems, both primary and the back-ups, responded by suspending automatic processing to ensure that no incorrect safety-related information could be presented to an air traffic controller or impact the rest of the air traffic system,” chief executive Martin Rolfe said.

“Our systems are designed to isolate the problem and prioritise continued safe air traffic control,” he added.

A British Airways plane is seen at London City Airport, London, Monday, Aug. 28, 2023. (AP Photo/Alberto Pezzali)

Aviation analytics firm Cirium said 790 departures and 785 arrivals were canceled across all U.K. airports Monday, and knock-on effects continued to affect dozens of passengers Tuesday.

At least 32 departures from Heathrow, Europe’s busiest air hub, were canceled Tuesday, and 31 arrivals axed.

British Airways, which operates the most flights to and from Heathrow, was the worst affected airline.

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Gatwick, London’s second-busiest airport, was similarly affected Tuesday.

Passengers stuck at airports across the U.K. described their frustration.

Katrina Harrison and her family, including her 1-year-old twin grandchildren, spent the night with no food or bed at Leeds Bradford Airport after their flight to Turkey was canceled Monday.

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“There were no hotels to stay in, we couldn’t get the car out of the car park … We haven’t slept. We tried to sleep on the floor but couldn’t. Luckily the children could sleep in the pram,” she said. “We’re hoping to get on a flight tonight but if it doesn’t happen tonight, we’ll have to go home.”