The failure of an air traffic control system has led to the number of flights in English airspace being restricted.
The UK National Air Traffic Service (NATS) has restricted the number of flights across the south of England to "maintain safety".
According to air transport expert David Kaminski-Morrow the restrictions are in place after the failure of a NATS conflict-alert system.
The technical issues were triggered by a "problem with a flight plan" entered into a system according to a spokeswoman for NATS.
"NATS is experiencing technical problems at our Swanwick control centre and we are working to restore full operations," said an official statement from NATS.
"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."
NATS provides air traffic navigation services to aircraft flying through UK controlled airspace and at several UK and international airports. Its website said it handled 7,086 flights on Monday.
The heaviest restrictions appear to be in airspace in the South West of England, the mainland from Southampton westwards and also including Jersey, with minor restrictions in the South East. Flights from UK's largest airport Heathrow are reported to be unaffected, while a Gatwick spokesperson said there are minor delays on some flights, particularly those to Jersey.
UPDATE (12.35pm) - NATS says it has corrected the problem and operations are now returning to normal.
The service is "working with the airports, airlines and Eurocontrol to clear the backlog of flights to ensure disruption is kept to a minimum".
Delays stemming from the fault have generally been limited to 20 minutes, it said.
Eurocontrol, the European Organisation for the Safety of Air Navigation, says restrictions will be in place until 4.40pm British time due to a backlog of air traffic.