Passengers were left stranded today when a technical problem at an air traffic control centre caused chaos at airports across the UK and Ireland.
The National Air Traffic Service (Nats) said the issue, which has led to flight delays and cancellations, was unlikely to be fixed until between 6.30pm and 7pm.
Thousands of people have been affected at major airports including Heathrow, Stansted and Gatwick.
Nats said the issue, related to an internal telephone system, arose at its control centre in Swanwick, Hampshire, in the early hours of the morning when the night-time operation failed to properly switch over to the daytime system.
A Nats spokeswoman said: " At night, when it's quiet, sectors of airspace are combined. As it gets busier in the daytime the sectors are split out again and additional control positions are opened to meet the traffic demand.
Because of the problem with the internal telephone system, it was not possible to open the additional control positions this morning, resulting in a significant reduction in capacity in some areas of UK en-route airspace.
" Safety has not been compromised at any time."
By midday on a Saturday, Nats would usually have handled around 2,000 flights but today that figure stood at 1,700.
The spokeswoman added: " We now understand what the problem is and our engineers are working hard to rectify the issues as quickly as possible. Everyone in our operation is continuing to work closely with our customers to manage the traffic and clear the backlog."
At Heathrow, the UK's busiest airport, there were 180 cancellations, split equally between arrivals and departures. A spokeswoman said: "We have around 25 minutes of arriving delays and 45 minutes of departing delays."
A Stansted Airport spokesman said flights had been subject to delays of between 30 minutes and three hours but added that the situation was improving throughout the day as aircraft flow restrictions were being lifted.
A spokeswoman for Gatwick Airport said: "We have had 86 delays so far, with the average delay being an hour. In addition, we have cancelled five flights.
"It is not a busy weekend for us and we are not running at capacity. No passenger likes to be delayed, and we do not like to delay passengers, but because we are not at capacity, we have had the space to deal with disruptions. We have got passengers on the planes and we are getting them away. We are doing our best to deal with the situation."
Many passengers took to Twitter to express their frustration at the delays.
Maeve Keegan, who was flying from Dublin to London today, was delayed by two-and-a-half hours. But she wrote on Twitter: "A couple on honeymoon missed connecting flight to Cancun! Nightmare."
Paralympic sailor Helena Lucas, who appeared to be one of the lucky ones, wrote: "Lots of angry people at Heathrow! Customer services is crowded with unhappy people! My flight looks on time fingers crossed!"
Take That singer Howard Donald was also caught up in the chaos.
He wrote on Twitter: "Control tower failure at Heathrow as left me stranded for 2-3 hours at dusseldorf. Anyone know any games besides eye spy?" (sic).
Twitter user @JustinLindberg wrote: "The entire air traffic control system has gone down at #Heathrow. No one knows when we will even board our flight. Wicked."
@PatBunton said on the site: "Stuck at Newcastle airport - a 3 1/2 hour delay - good luck to all @Oldfield_RFC players - best laid plans to get back and play in tatters."
Other airports experiencing problems included Dublin, Cardiff, Luton, Newcastle, Edinburgh, Glasgow, Bournemouth, Manchester and Liverpool.
Independent aviation analyst Chris Yates said: "It's going to be a day of frustrations and the knock-on effects are going to last for the whole day because of the backlog of planes. It will be a tough day for everybody."
Passengers were advised to check with their airline for the latest situation.
The Association of British Travel Agents (Abta) said: " Customers due to travel today should liaise with their airline to establish if there are any changes to their flight arrangements.
"Furthermore, customers should be prepared for potential delays and are advised to carry essential items such as medication in their hand luggage."