THE GOVERNMENT demanded an explanation last night after an air traffic control failure led to chaos in the skies above London.

Dozens of flights were cancelled and many others delayed after a computer failure at the headquarters of air traffic control company Nats.

For a time there were no flights able to take off or land at some UK airports and although Nats were able to resolve the problem by around 4pm, delays and cancellations were expected to drag on into the evening.

Transport Secretary Patrick McLoughlin said the disruption was unacceptable.

He went on: "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."

The problem at Nats' state-of-the-art £700 million centre at Swanwick is by no means the first glitch at the Hampshire centre.

Having resolved the fault, which started around mid-afternoon, Nats said: "It will take time for operations across the UK to fully recover so passengers should contact their airline for the status of their flight.

"We apologise for any delays and the inconvenience this may have caused."

Heathrow said 50 flights had had to be cancelled at the west London airport and warned that this figure could rise and that delays could persist. A spokesman added that as a result delays were expected to flights over the weekend.

The foul up had implications for other airports as well.

Large queues formed in front of check-in and customer services desks at Heathrow Terminal Five.

Worried travellers stood waiting for updates in front of flight information boards and television screens.

Some complained that airport staff could not tell them what was happening with their flight or what had caused the problem.

Susan Atkinson and husband Michael Alcock, from Trumbull in Connecticut, US, anxiously awaited news on their 8pm flight to Manchester.

Investment bank worker Ms Atkinson, 53, said she was "disappointed" at the delay because she had not seen her family in Lymm, Cheshire, for two years.

"I just want to get home to see my father and sister. I couldn't make it last year because of ill health.

"Hopefully we can fly tonight, we don't have anyone to stay with here in London."

Retired Mr Alcock, 70, said: "I would say for now the word to describe how we're feeling is disappointed.

"If the hours go by without anything changing that could become more hostile feelings, especially since we don't really know what is going on."