Flights across the UK have been delayed or cancelled by an air traffic control system problem.
Thousands of passengers at airports, including Heathrow and Gatwick, have been affected by the "technical problem" at southern England's main air traffic control centre in Swanwick, Hampshire, Sky News reports.
A Heathrow Airport spokeswoman said: ""We are currently experiencing some delays."
While Gatwick tweeted: "Due to air traffic control systems issues some flights may be delayed. Please check with your airline."
The BBC reports that the National Air Traffic Control Service (Nats) said its Swanwick centre was having "difficulty switching from night time to daytime operation".
It apologised for the delays and added: "Engineers are working to rectify the problem as soon as possible."
Other airports that have reported delays are Luton, London City, Exeter, Edinburgh, Manchester, Birmingham, Southampton and Newcastle.
Low-cost airline Ryanair warned there will be "significant flight delays and possible cancellations".
A Stansted spokeswoman told The Guardian: "There are some issues with the air traffic control system.
"The result of it is that, at the moment, 20 per cent 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."
Passenger Daisy McAndrew told Sky News she had been caught in the "unholy mess" at Gatwick as she tried to fly to Barcelona for work. She said: "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."
"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."