Almost 40 flights have been cancelled at Heathrow on Saturday.

While none of the airlines flying to India from the airport reported any major disruptions, it has now emerged that the problem may be repeated because software used to control flights by Britain's controllers dates back to 1960s.

National Air Traffic Services (Nats) said a technical fault in the flight data system at its Swanwick centre, in Hampshire had caused the problem.

This resulted in widespread disruption at airports around the UK on Friday. It is the latest in a string of computer  problems that have plagued Nats in recent years at its south England headquarters.

A consultant who has worked for Nats told it knew its software needed to be replaced a decade ago but will be relying on the 1960s programmes for another two years.

Martyn Thomas, visiting professor of software engineering at the University of Oxford, said, "The National Airspace System that performs flight data processing was originally written for American airspace in the late 1960s."

"It wasn't designed to cope with the volume of air traffic we have today, or to interface with modern computer software," Thomas added.

Professor Thomas said the system was written using a now defunct computer language called Jovial, meaning Nats has to train programmers in Jovial just to maintain the antiquated software.

UK Transport Secretary Patrick McLoughlin demanded answers from Nats after airspace over London and the south-east of England was closed for nearly 36 minutes, leading to a backlog that could take until this afternoon to clear.

McLoughlin said, "Any disruption to our aviation system is a matter of the utmost concern, especially at this time of year in the run up to the holiday season. Disruption on this scale is simply unacceptable and I have asked Nats for a full explanation of this evening's incident. I also want to know what steps will be taken to prevent this happening again".

Heathrow Airport has warned of knock-on effects on Saturday and asked passengers to check their flight status.

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