Nats has identified and corrected the technical problem that has been affecting air traffic control systems in the south of England.
They have released this statement
Operations are now returning to normal and we are working with the airports, airlines and Eurocontrol to clear the backlog of flights.
Outbound delays from the UK have generally been limited to 20 minutes.
We regret any inconvenience this technical problem has caused, however our first priority will always be the safety of the flying public."