“The system has been restored and we are in the process of returning to normal operations,” the spokeswoman said.

However, it may be some hours before the affected airports, including Heathrow, Europe’s busiest, can work through the disruption from the incident. The problem effectively stopped flights out of most of the U.K. for the best part of an hour, and Heathrow alone handles roughly 1,300 flights a day.

The spokeswoman said it would be up to the individual airports and airlines to work out how they clear the backlog of flights arising from the incident.

Asked about the possibility of a cyberattack, the spokeswoman said it was too early to rule anything out, but added that there was no information immediately available to suggest that had been the cause.

NATS said earlier that a “technical problem” had affected its center at Swanwick in Hampshire, southern England, from which it controls all flights over the London Flight Infomation Region. The London FIR covers all of the major airports in England and Wales.

Heathrow is one of the most sensitive pressure points in the U.K.’s economy, and has been the target of terrorist attacks in the past, notably when the Irish Republic Army launched mortar bombs at its runways in 1994.

NATS in a statement advised anyone planning to travel through the disrupted region to check the status of their flight with their airline.

The spokeswoman declined to confirm a tweet from Heathrow Airport’s Twitter feed alleging a power outage at the center.

Swanwick has had technical issues occasionally in the past, most recently in December last year, when a problem with its telephone system caused it to run at reduced capacity by over 10% over a period lasting several hours.