A "technical problem" at the air traffic control centre is causing delays at airports across the UK.
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".
National Air Traffic Services (Nats) said the issue stemmed from its control centre in Swanwick, Hampshire.
A spokesman said: "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."
A spokeswoman for Dublin Airport said: "I understand that there are some flight delays as a result of air traffic control issues in the UK."
The Stansted spokesman said: "There are restrictions on the air space 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.
"There are no problems with arrivals at the moment."
Passengers are advised to check with their airline for the latest situation.
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."
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 spokesman for Cardiff Airport said all flights this morning had been affected.
Incoming flights from Glasgow, Dublin and Amsterdam are delayed with no estimated time of arrival.
Passengers travelling from Malaga to Cardiff were expected at 8.35 but this has been delayed to 10.20.
Those leaving Cardiff Airport are also experiencing delays, with a flight to Paris delayed with no estimated time of arrival.
A flight to Malaga has been delayed from 9.10 to an estimated take-off at 10.55, with one to Amsterdam delayed from 10.25 to an estimated time of 11.10.
A spokesman for Cardiff Airport said: "Both (departures and arrivals) have been delayed because of the knock-on effect of the late arrivals of the service.
"We are currently looking at an estimated 1hour 40 minutes delay."
There have been cancellations at Heathrow Airport for flights scheduled to leave this morning.
A Heathrow Airport spokeswoman said: "We are currently experiencing some delays."
Flight information on the airport's website showed several departures to both domestic and international destinations had been cancelled.
A spokeswoman for London City Airport said she believed it was affected in the same way as all London airports, and said a statement would be made as soon as possible.
A spokesman for Luton Airport said it was "experiencing delays across the board".
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."