Airline passengers suffered a day of delays and frustration as a telephone system glitch at air traffic control caused major disruption at airports across the UK and Ireland.
Thousands of people were caught up in the chaos, which hit major airports including Heathrow, Stansted and Gatwick.
While the issue has now been fixed, the disruption continues. Some Sunday flights are also expected to be affected.
Eurocontrol, the European organisation for air navigation safety, said it had been working with the National Air Traffic Service (Nats) and its counterparts in Holland and France during the day.
It said about 1,300 flights, nearly 8% of all traffic in Europe, had been "severely delayed."
Computers in the traffic control center crashed on Saturday morning when operators tried to switch from night to day operations. This caused a communication problem with the centre's internal telephone system.
"This is a very complex and sophisticated system with more than a million lines of software. This is not simply internal telephones, it is the system that controllers use to speak to other ATC agencies both in the UK and Europe and is the biggest system of its kind in Europe," a statement by the agency said.
Our apologies again to everyone who was affected by today's outage.
Nats stressed that safety was not at risk at any time.
The issue was finally resolved at 7:30 pm, nearly 12 hours after problems started at the £623 million state-of-the-art air traffic control centre at Swanwick in Hampshire.
Lewis Vaughan Jones reports:
Heathrow Airport was the worst affected, with 228 cancellations.
At Stansted Airport, flights were delayed throughout the whole day. The average delay was two hours, a spokesperson said.
Other airports affected were Dublin, Cardiff, Belfast, Glasgow, Edinburgh, Bournemouth, Manchester, Birmingham, Southampton, Luton, London City, Exeter and Newcastle
Many frustrated passengers voiced their anger on Twitter:
@british_airways poor service-flight cancelled, no warning/explanation.4 staff for 200 passengers at Vienna airport. Bad end to honeymoon.
Closest I'm getting to Milan today is cafe Nero at heathrow. All flights cancelled :( cheese panini n espresso please. Ciao #egg
It wasn't just the passengers who was angry with Nats.
Ryanair has hit out at the UK's Civil Aviation Authority (CAA) for failing to have a contingency plan in the event of an air traffic control (ATC) failure.
In a statement, the airline said: "While we acknowledge problems can occur, where is the contingency? It's simply not good enough and the CAA needs to act now."
Despite the issue being resolved, disruption to services across the country continues. Heathrow Airport officials said they "still advise passengers to check with airlines prior to travelling."