Technical problems are affecting the National Air Traffic Control centre at Swanwick and the organisation is working to restore full operations.
This has not resulted in the closure of UK airspace or the suspension of all flights in or out the UK, Nats said.
Aircraft are still moving, but restrictions are in place to ensure safety.
“To maintain safety, Nats is restricting the number of aircraft flying across the south of England and those taking off from airports,” a statement said.
“We regret any inconvenience this may cause, however our first priority will always be the safety of the flying public.
“Nats is taking every step to restore services and has contingency plans in place to minimise any disruption.”
More information will be released as it becomes available, the air traffic control service added.