By Jonathan Davies

More than 10,000 passengers were affected at Heathrow alone by the computer glitch which forced London's air space to be closed on Friday.

Transport secretary Patrick McLoughlin said the closure of London's air space caused a collective 16,000 minutes of delays.

He described the failure as "unacceptable", but fought to defend the National Air Traffic Services where the glitch happened.

"The average delay this year in Nats is 2.5 seconds per flight, whereas the rest of Europe we're taking about 30 seconds," he said.

The transport secretary also pointed to a telephone failure in December 2013 which led to 126,000 minutes of delays.

Nats chief executive Richard Deakin rejected criticism from business secretary Vince Cable, who accused the company of"skimping on large-scale investment" and being "penny wise and pound foolish".

He said Nats is set to invest £575m in its systems over the next five years.

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