Flights are being delayed across the UK and Ireland because of an air traffic control centre fault.
Affected airports include Heathrow, Stansted, Cardiff, Dublin, and Glasgow.
The National Air Traffic Control Service (Nats) said its Swanwick centre, in Hampshire, was having "difficulty switching from night time to daytime operation".
It said the problem will not be fixed until between 1800 GMT and 1900 GMT with delays likely to continue beyond.
Operations director Juliet Kennedy told the BBC she did not think the problem had happened before.
BBC transport correspondent Richard Westcott said Nats' internal phone system had broken down, which "meant controllers in the same room as each other couldn't pass on important data to each other".
He said it was a totally different issue to the software problem Nats suffered earlier this summer.
At midday, air traffic controllers had dealt with 1,700 flights rather than the usual 2,000 on a normal Saturday.
Heathrow had cancelled 60 flights by 09:45 GMT, with these split between arrivals and departures.
Belfast, Edinburgh, Manchester, Birmingham, Southampton, Luton, London City, Newcastle, Exeter and Bournemouth are among other airports that have reported delays, asking passengers to check with their airlines.
Stansted Airport said flights were subject to delays, while Gatwick said 20% of its departures had been delayed, with passengers being warned they may have to wait for "a couple of hours".
In other developments:
- British Airways said the technical problems had already led to flight cancellations and warned that they "will cause delays to some flights"
- EasyJet said though the majority of the morning flights had departed, "severe delays" and possible cancellations could be expected later
Travel reporter Simon Calder told BBC Breakfast early morning delays could lead to cancellations later in the day.
He said there were "lots and lots of delays" at Stansted, while Heathrow Airport had "long delays" of up to 40 minutes in departures.
Ms Kennedy said the problem is capacity, as the control centre is still running a reduced night time service.
She said: "We just can't manage as many flights as normal," adding that it is up to individual airlines to prioritise who goes where.
At Stansted, Alena Kontza's is stuck on a Ryanair plane that has been delayed for three hours. She told the BBC passengers had been given "absolutely no information" and "it had been nothing less than shambolic".
"People are really aggravated, children are crying, people want to leave, people want to change to different planes, it's an absolute nightmare," she said.
William Paton, from London, has been delayed at Heathrow. He told the BBC: "Having been on a cancelled flight to Aberdeen from City last night, I was put onto a red eye this morning... and am still on it, with delays of four hours estimated, according to the captain."
'Backlog of planes'
Nats said the technical problem at its Swanwick control centre started in the early hours of Saturday morning.
It said in a statement: "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."
It said the glitch meant it had not been possible to reconfigure the voice communications system to split out the sectors for the daytime traffic in some areas of the UK airspace.
"Safety has not been compromised at any time, and we sincerely apologise for any inconvenience being caused to passengers," it added.
Independent aviation analyst Chris Yates said it was going to be a "day of frustrations".
He said that passengers due to arrive at UK airports from overseas could find themselves diverted elsewhere.
"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.