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 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.
All airlines at London airports are suffering delays because of an air traffic control system issue. We are doing all we can to minimise disruption to our flights, but there will inevitably be a significant number of short-haul cancellations at some UK and European airports today.Customers are advised to check ba.com for latest flight information before travelling to the airport. Customers on cancelled flights will be able to claim a full refund or be rebooked on to alternative flights. Given that Heathrow is the world's busiest two-runway airport and Gatwick is the world's busiest single-runway airport, there will be problems for all airlines as a result of the ATC failures."