UK air traffic management agency NATS is experiencing technical problems at its Swanwick control centre, which has forced it to restrict flights across the south of England.
The agency said it was working to restore full operations and was keen to emphasise that the fault has not resulted in the full closure of UK airspace or the suspension of all flights in or out of the UK.
"However, to maintain safety NATS is restricting the number of aircraft flying across the south of England and those taking off from airports," the agency said in a statement.
"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."
Twitter users are already beginning to complain about flights being delayed because of the systems problem.
Although NATS hasn't yet confirmed the cause of the problem, it recently awarded Lockheed Martin a seven-year contract extension to continue supporting its air traffic management systems at its Swanwick operation centre.
The deal follows a five year £80 million contract NATS awarded to Lockheed in March 2008. Lockheed looks after systems architecture, systems engineering, software development, systems integration and laboratory support.
In the last financial year NATS controllers guided 2.2 million flights over the UK and northern Atlantic.