A technical failure at Swanwick control center, according to NATS, formerly the National Air Traffic Services, is behind the issue.
NATS tweeted, "Airspace is open. We're restricting traffic volumes in accordance with capability we currently have in our system.
NATS can confirm that a technical problem has been reported at Swanwick air traffic control center. We apologize for any delays and our incident response team has been mobilized. Every possible action is being taken to assist in resolving the situation and to confirm the details," a statement from NATS read.
All London airspace is closed due to a "computer failure," according to an earlier statement from Eurocontrol, the European Organization for the Safety of Air Navigation.
