Passengers have described being trapped in an "unholy mess" after flights at airports across the UK have been thrown into chaos by a "technical problem" at the air traffic control centre.
Affected airports currently include Heathrow, Stansted, Cardiff, Dublin, and Glasgow, in a situation described as "embarrassing" for the UK's air traffic control system.
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.
"I have never heard of an example where every single plane is grounded - it's quite eerie when I look out of the window to see the tarmac in Gatwick, normally so busy, and also the sky above Gatwick which is normally busy - completely static, there's nothing moving."
A Stansted Airport spokesman said all of its departing flights were subject to delays of between 30 minutes and two hours, while Gatwick Airport said 20% of its departures had been delayed, with passengers warned they could wait for "a couple of hours".
NATS air traffic control said in a statement: "Due to a technical problem at Swanwick we are currently experiencing some difficulty switching from night time to daytime operation.
"This may result in some delays for which we apologise. Engineers are working to rectify the problem as soon as possible."
Travel journalist Simon Calder said the technical glitch was a "disaster".
He told Sky News: "The south-east of England is the busiest airspace in the world. London handles far more passengers than anywhere else including Paris, New York, Tokyo and so on.
He explained that the lack of spare capacity at airports like Heathrow means things get "very messy, very quickly" and airlines are forced to cancel flights to create firebreaks, which allow the system to keep running.
He said there was "no way" that passenger safety would be compromised due to the ongoing problems but that airlines were facing losses of millions of pounds.
He added: "If you are flying today, then good luck, if you're only delayed you're in a pretty good place."
Passengers are advised to check with their airline for the latest situation.