Image copyright PA Image caption There were delays to many flights from Heathrow on Friday

Airports around the UK have been returning to normal, although almost 40 flights were cancelled at Heathrow, the day after a computer failure at the national air traffic control centre.

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.

Transport Secretary Patrick McLoughlin said the situation was "unacceptable".

The Swanwick centre was restored to "full operational capacity" on Friday afternoon, Nats said.

Heathrow Airport said 38 flights due to arrive or take off before 09:30 GMT on Saturday had been cancelled but the subsequent schedule was "back to normal".

A Heathrow spokesman said the cancelled flights could not be rescheduled because the airport ran at 98% capacity. He said passengers would be rebooked on other services.

Media playback is unsupported on your device Media caption Nats managing director of operations Martin Rolfe told the BBC that the cause of the system failure was still being investigated

British Airways said there may be "some disruption" to its Heathrow flights on Saturday.

It said Gatwick and London City flights were expected to operate as normal but advised all passengers to check the status of their flight before leaving for the airport.

Gatwick Airport said it would be operating a full service on Saturday although there would be "some backlog".

Stansted said all its flights were running on schedule.

Many other airports are also due to run their scheduled Saturday flights on time, according to their websites.

Cause unknown

The glitch caused many delays at Heathrow and Gatwick on Friday, where departing flights were grounded for a time.

Dozens of arrivals and departures at airports across southern England, and as far north as Aberdeen and Edinburgh, were delayed and cancelled.

Image copyright PA Image caption Many of those who travelled to Heathrow on Friday were in for a long wait

Image copyright PA Image caption It was a trying experience, particularly for those with young children

Image copyright PA Image caption Some passengers were eventually able to catch flights

Many passengers had to stay in hotels overnight because of rescheduled flights.

Bertie Rowell from west London was supposed to be flying to Paris for a celebratory weekend with his girlfriend but said his flight from Heathrow on Friday afternoon had been cancelled.

"We had to stand for two-and-a-half hours in a queue with no explanation of what was happening," he said.

"We stayed in a hotel hoping to go on standby this morning. At 6am we went back to the airport only to be told we were better off going home.

"We were looking forward to sipping coffee and people-watching in Paris. Now I'm back at home wondering what I can do to make up for the disappointment."

Swanwick air traffic control centre

Image copyright PA

Swanwick controls the 200,000 square miles of airspace above England and Wales, cost £623m to build, and employs about 1,300 controllers.

But the facility, which handles more than 5,000 flights every 24 hours, has had a troubled history.

It opened in 2002, six years after its planned commissioning date - a delay which Nats said was due to problems with the software used to power its systems.

Almost a year after it opened, a senior air traffic controller raised concerns with the BBC about health and safety standards and complications with radio communications - which he said cut out erratically.

Technical problems and computer faults hit flights in 2008 and again last summer. And, in December 2013, problems with the internal telephone system then caused further delays.

Friday's problems came a year after a telephone failure at the Hampshire control room caused huge disruption - one of a number of technical hitches to hit the part-privatised Nats since the centre opened in 2002.

Nats managing director Martin Rolfe has ruled out both a computer hack and a power outage as possible causes of Friday's problem, but says the precise cause has yet to be established.

He said the error occurred in the flight data part of the system, where flight plans are stored.

BBC correspondent Andy Moore, at Heathrow, said the issue only lasted for between 30 minutes and one hour but caused chaos because the UK's air traffic control system runs at 99% capacity, giving little scope for managing disruption.

Mr McLoughlin said any disruption to the nation's aviation system was 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... I also want to know what steps will be taken to prevent this happening again."

Michael Dugher MP, Labour's transport spokesman said: "It's simply not good enough that a computer glitch has caused such disruption to so many families and businesses who were relying on flights."

He added: "The government needs to get to the bottom of what happened and ensure that something like this can't happen again."

'Full pelt'

Martin Clipp, a former senior operations manager at Nats, told BBC Radio 4's Today programme: "You can't build absolute resilience... in terms of the technology that backs up the systems.

"The system runs pretty much at full pelt. That means as soon as the slightest thing goes wrong the ripples go out enormously."

However, Mr Clipp said Nats was being required by the Civil Aviation Authority to cut costs, which had led to redundancies this year.

"There is no risk in safety but there is risk in service continuity," he said. "You get what you pay for."

The Independent's travel editor, Simon Calder, said Friday would be an expensive incident for the airlines, estimating they would pay £2m to £5m in compensation.

"If a flight is delayed, even if it's nothing to do with the airline that's caused it, the airline is responsible for looking after the passengers."