The British government said Tuesday that a breakdown at the nationwide air traffic control system that saw hundreds of flights delayed and canceled was not caused by a cyberattack.

Transport Secretary Mark Harper said the problem was caused by a “technical fault” at flight control operator National Air Traffic Services. The company said the outage had hit its ability to process flight plans automatically, meaning that for several hours the plans had to be input manually, a much slower process that meant fewer flights could take off and land.

Harper said it was the worst incident of its kind in almost a decade. He told the BBC that “it is going to take some days to get people back to where they should be.”

The problem hit on a late-summer holiday Monday that is one of the busiest days of the year for air travel. Aviation analytics firm Cirium said 790 departures and 785 arrivals were cancelled across all U.K. airports on Monday.

Dozens of flights were canceled at Heathrow, Europe’s busiest air hub, which warned of “knock-on impacts.” At least 32 departures from Heathrow were canceled on Tuesday, and 31 arrivals axed.

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

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 one-year-old twin grandchildren, spent the night at Leeds Bradford Airport after their flight to Turkey was axed on Monday.

“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.”