Ongoing delays

NATS' Flight Data Processing (FDP) system, based at West Drayton, went down at O930 BST.

The system produces paper slips which tell air traffic controllers each individual aircraft's height, route, destination and contact information, allowing them to direct the planes correctly.

When it is not working, controllers have to produce these forms by hand, which is slower so it can cause delays.

It was restored at 0950 BST, by which time hundreds of planes at airports across the UK had been grounded.

Passengers planning to fly today are being advised to check in as normal.

At Heathrow, 100 flights were directly affected by the fault as no planes were allowed to take off for 40 minutes.

Knock-on delays of about 30 minutes were expected on flights throughout the day.

A spokesman for Manchester Airport said: "At one point we had 30 flights to destinations across Europe delayed.

"That has now been cut to nine, which we are working to slot back into the schedules, but all other flights are leaving on time as normal."

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'Sincere apologies'

Ryan Martinez, from Nottingham East Midlands Airport, said: "Things are getting back to normal now, though there is going to be the odd half hour delay through the day."

Ian Hall, NATS's director of operations, said: "We sincerely apologise to those who have been inconvenienced this morning.

"We have taken every step to avoid problems, but are aware that in maintaining ageing and complex systems we could experience difficulties."

A spokeswoman for the service said it was too early to tell whether the problem was caused by the same fault that saw hundreds of flights delayed in June 2004.

NATS plans to introduce a new FDP system as part of its £1bn modernisation programme.