The computer systems of the United Kingdom's primary air traffic control center have been restored and the country's National Air Traffic Services is ramping operations back up after an unknown failure grounded and diverted the majority of the flights destined for or departing from London's Heathrow Airport.

Previous reports indicated the NATS' system failure was the result of a power outage, though the air guidance company says its preliminary investigation has ruled that out. Air space capacity, however, remains restricted.

"Following a technical failure at Swanwick, the system has been restored," states NATS. "However, 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 apologize for any delays and the inconvenience this may have caused. We are investigating the cause of this fault but can confirm that contrary to some reports, it was not due to a power outage."

NATS confirmed in an earlier statement that U.K. airspace was never closed as a result of the system failure and a government source told Reuters the issue didn't appear to be the result of a security threat to the U.K.

"U.K. airspace has not been closed, but airspace capacity has been restricted in order to manage the situation," stated NATS. "We apologize for any delays and our incident response team has been mobilized. Every possible action is being taken to assist in resolving the situation and to confirm the details."

At least 14 airports were directly affected by Heathrow's massive delays. But as an airport that sees an average of 1,286 flights daily, the full scope of the NATS system failure is still unknown.

So far, 66 flights at Heathrow have been cancelled. That number could increase as NATS continues to return to full capacity. Heathrow is the U.K.'s busiest airport and the busiest for passenger traffic in Europe.

"Disruption on this scale is simply unacceptable and I have asked NATS for a full explanation of this evening's incident," says Patrick McLoughlin, the U.K.'s transport secretary.

NATS' system failed at its air traffic control center in Swanwich, a sister facility to the organization's other location in Prestwick, Scotland. The U.K. organization's presence in the world extends beyond its two control centers, into more than 30 countries.

NATS has contracts with the U.S. and provides consultancy services to Canada and countries in the Caribbean. The government-contracted organization also offers consultancy services to countries in Africa, the Middle East and the Asia-Pacific region.