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.
Hundreds of flights were cancelled or delayed after the problem arose at the National Air Traffic Service (Nats) centre in Swanwick, Hampshire.
Thousands of people were caught up in the chaos, which hit major airports including Heathrow, Stansted and Gatwick. Passengers are also expected to be affected as knock-on effects hit flights on Sunday.
Nats' night-time operating system, which combines sectors of airspace for when it is less busy, did not properly switch over to the daytime system, causing a communication problem with the centre's internal telephones. They stressed that safety was not at risk at any time.
At 7.30pm tonight, a Nats spokesman said: "The problem that arose this morning with the ground communications system in the area control operations room at Nats Swanwick has now been resolved and operations are returning to normal.
"The technical and operational contingency measures we have had in place all day have enabled us to deliver more than 80% of our normal operation.
"The reduction in capacity has had a disproportionate effect on southern England because it is extremely complex and busy airspace and we sincerely regret inconvenience to our airline customers and their passengers.
"To be clear, 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.
"This has been a major challenge for our engineering team and for the manufacturer, who has worked closely with us to ensure this complex problem was resolved as quickly as possible while maintaining a safe service."
The problem occurred when the 23 controllers on a night shift at Swanwick handed over to the 125 controllers on the day shift at about 6am. It was not until 7.30pm that Nats announced the problem had been fixed.
Nats said that by 4pm they had handled 2,576 flights compared with the 2,905 flights that were dealt with at the same time last week, meaning that they were operating at about 88% of normal capacity.
Ryanair called for action to prevent it happening again. In a statement, the airline said: "Ryanair is calling on the Civil Aviation Authority to intervene and prevent further chaos for thousands of passengers affected by this ATC failure. While we acknowledge problems can occur, where is the contingency? It's simply not good enough and the CAA needs to act now."
Heathrow Airport was the worst affected, with 228 cancellations - 112 in arrivals, and 116 departures, with most being short-haul flights. A spokesman for the airport said the cancellations represented 15% of their usual daily total of 1,300 flights going in and out of the airport.
Some passengers at the airport dozed on the floor while others lined up in long queues to rebook their flights, with customers reporting they had to wait for up to five hours to speak to a representative from their airline.
Gatwick Airport said "there may be delays to flights this evening" and urged travellers to check with their airline. The airport said there was spare capacity today however, which "allowed for flexibility and resilience within the schedule".
A spokesman for Stansted Airport said they had continued to experience delays throughout the day, which ranged in time from 10 minutes to four hours. The average delay was two hours, he said, adding that flights were expected to operate as normal tomorrow.
Many passengers expressed their anger on Twitter and spoke of "chaotic scenes" at airports.
Danny O'Donoghue, lead singer with The Script and a judge on BBC talent show The Voice, wrote: "Stranded @ the airport , anyone got any suggestions on passing time ...."
Twitter user @simonhartley wrote: "Darn NATS. Stuck in Madeira airport with at least five hour delay home so far :(."
And @MMorrissey wrote: " My flight to NY delayed but chaotic scenes here. In 20 yrs of business travel, never seen Heathrow in this state when there's no snow!!"
In developments at airports around the rest of the UK:
:: A Manchester Airport spokesman said the airport had been "only minimally affected", with "a handful of flights" hit during the day. Operations there are now back to normal.
:: Leeds Bradford, Doncaster Robin Hood and Newcastle airports all reported some minor delays, with flights to London airports the worst affected.
:: Belfast International Airport and Dublin Airport reported disruption, with a number of cancellations.
:: A spokeswoman for Birmingham Airport said around 60 flights had been affected by the glitch during the course of the day, with the majority delayed by under an hour.
A Department for Transport spokeswoman said: "This is an operational issue relating to Nats' own systems and we know they are working very hard to rectify the problem and to minimise the level of disruption to the aviation community and their customers.
"Nats are continuing to keep the airlines, airports and department fully informed of their progress towards resolving the issues which have arisen today. We recommend that passengers should check the status of their flight with their airline before travelling to the airport."
Eurocontrol, the European organisation for air navigation safety, said it had been working with 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".
In a statement, Eurocontrol said: "This incident highlights once again the importance of the robustness of the technical systems supporting air traffic management and the need for contingency planning at network level to minimise the impact of any failures on the travelling public."
The Civil Aviation Authority (CAA) said: "The CAA is in close contact with Nats to ensure they are managing this operational issue effectively, and they are working to resolve the situation swiftly, while ensuring that the service remains safe at all times."
The authority said customers were entitled to assistance from their airline if they had been delayed for several hours, including being given food and drink, often provided in the form of vouchers.
For short-haul flights, passengers should receive help if the delay is more than two hours. That rises to three hours for medium-haul flights of 1,500km to 3,500km, or of longer than 1,500km within the EU. For long-haul flights, passengers should receive assistance once the delay is over three hours.
The CAA advised people to contact their airline to discuss individual cases.
More than 100 Ryanair flights to and from the south of England have been affected, with 10 cancelled so far. The airline said: "We apologise to affected passengers, however these circumstances are entirely beyond our control."
A spokeswoman for British Airways said: "We are organising hotels for customers when appropriate. In addition, customers on cancelled services of course have the opportunity to claim a refund or rebook.
"We would like to apologise to our customers for the inconvenience caused.
"We are doing all we can to minimise disruption to our flights, but inevitably these issues have led to a significant number of short haul cancellations at some UK and European airports today."
Virgin Atlantic urged customers to check their flight status before leaving home, and thanked them for their patience.
EasyJet asked passengers to check in as normal but to use the flight tracker service on their website for updates. A statement on their website read: "Although outside of our control, we're sorry for any delays you experience today."