Flights have been delayed and cancelled at airports across the UK and Ireland because of a technical problem at an air traffic control centre.
National Air Traffic Services (Nats) does not currently know how long it will take for the situation to be rectified.
Thousands of people have been affected at major airports including Heathrow, Stansted and Gatwick.
Nats said the issue arose at its control centre in Swanwick, Hampshire, in the early hours of the morning when a computing glitch meant the night-time operation failed to properly switch over to the daytime system.
Heathrow had cancelled 60 flights by 9.45am, with that figure split roughly equally between departures and arrivals.
A Heathrow Airport spokeswoman said: "Due to a technical issue with air traffic control, flights from many UK airports, including Heathrow, are subject to delay and cancellation.
"If you are flying today you should check the status of your flight with your airline. We are sorry that passengers have experienced disruption to their journeys."
A Nats spokesman said: " Due to a technical problem at Swanwick we are currently experiencing some difficulty switching from night-time to daytime operation. At night, when it's quiet, we can combine sectors of airspace. When it gets busy in the daytime, we split the sectors out again. The voice communications system is configured to enable this to happen.
"We experienced a technical problem in the early hours of this morning, which means that it hasn't been possible to reconfigure the voice communications system to split out the sectors for the busier daytime traffic in some areas of the UK enroute airspace.
"Engineers are working to rectify the problem as soon as possible, but this is resulting in some delays. Safety has not been compromised at any time, and we sincerely apologise for any inconvenience being caused to passengers."
A Stansted spokesman said all of the Essex airport's departing flights were subject to delays of between 30 minutes and two hours, while Gatwick said 20% of its departures had been delayed, with passengers warned they could wait for "a couple of hours".
The Stansted spokesman said: "There are restrictions on the airspace and the flow of aircraft. Our first departures go at 6am, so that's when the problems started."
A spokeswoman for Gatwick Airport said: "There are some issues with the air traffic control system.
"The result of it is that, at the moment, 20% of our flights are being delayed, by anything up to a couple of hours, but we're getting people moving and getting them away."
A spokeswoman for Dublin Airport said it was experiencing delays, with all flights at Cardiff Airport affected this morning.
London City Airport said about 50% of its flights had been disrupted, while Luton said inbound flights were unaffected but outbound flights had been hit.
Scotland's two biggest airports reported delays. A spokesman for Edinburgh Airport said the Nats technical fault had led to delays on southbound flights and he asked passengers to contact their airline if they are due to travel today.
Glasgow Airport tweeted: "A technical issue with air traffic control is resulting in some flight delays. Pls check with your airline for specific flight info."
Bournemouth Airport in Dorset and Newcastle Airport were also affected.
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."
Mr Yates said Swanwick controls all aircraft over the South of England, meaning thousands of passengers would be affected.
"There are contingency plans in place whenever this happens," he said. "Many of the long-haul flights, coming from China, India, the US and so on, passengers sitting on those planes may find themselves diverted to continental airports.
"But it's going to be a long wait for them. When the system kicks back in and starts working, there will be a backlog of flights.
"For those waiting to fly out, it's going to be a case of sitting around the airport terminal until things get back to normal."
Passanger Daisy McAndrew said she had been caught in the "unholy mess" at Gatwick as she tried to fly to Barcelona for work.
"As ever, staff have been fantastic but they know nothing other than the fact it is going to be a very, very long delay - very frustrating," Ms McAndrew told Sky News.
"And also, it's embarrassing, isn't it? When you look around a lot of people on my plane are not British, they are flying British Airways, they are probably trying to get back to Spain and they will inevitably be thinking this is something that could have possibly been prevented.
"It doesn't show our air traffic control system or our travel system in a good light."
Passengers are advised to check with their airline for the latest situation.
Virgin Atlantic wrote on Twitter: "Due to issues with UK air traffic control this morning, we are experiencing some delays."
On its website, Ryanair apologised for any inconvenience, writing: "Ryanair has been advised of an equipment failure within UK air traffic control which will cause significant flight delays and possible cancellations."
EasyJet advised passengers to check in for flights as normal "as the situation can improve", but suggested they refer to the airline's flight tracker page for updates.
A message on its website read: "Although this is beyond easyJet's control, we apologise for any inconvenience that you may experience today."
The problem is unlikely to be fixed until 2pm at the earliest.
The Association of British Travel Agents (Abta) said: "Nats air traffic control has advised that, due to air traffic control telephone equipment failure at Swanwick, they are currently experiencing some difficulty switching from night-time to daytime operations.
"T echnicians are working to try to resolve the issue but have advised that this is unlikely to be before 2pm today.
"Delays and flight cancellations at some UK airports are being experienced as a result.
"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."