The chaos that hit British airports yesterday was sparked by an unprecedented systems failure, it was disclosed today.
Air traffic control company Nats has given its first detailed explanation of the problem at its national centre in Swanwick, Hampshire, that saw dozens of flights cancelled and delayed, including those between Scotland and London.
It said: "Swanwick controller workstations provide a number of tools and services to the controller to enable them to safely control a high volume of air traffic.
"In normal operations the number of workstations in use versus in standby fluctuates with the demands of the traffic being controlled.
"In this instance a transition between the two states caused a failure in the system which has not been seen before. The failure meant that the controllers were unable to access all of the data regarding individual flight plans which significantly increases their workload.
"Our priority is to maintain a safe operation for the flying public; consequently when the failure occurred we immediately took steps to reduce the traffic into and out of the UK network.
"The controllers had a full radar picture and full communications with all aircraft at all times during the incident and at no time was safety compromised in any way."
Nats said it understood the problem was connected to a number of workstations "in a certain state" combined with the number of "air space sectors" open.
Officials restricted air space in response to the issue, leaving flights at some airports grounded yesterday.
Nats declared that its systems were back to full operational capacity last night but a knock-on effect has been seen at airports today.
Heathrow said 38 flights had been cancelled before 9.30am.
In a statement, the airport said: "Following yesterday's technical problem at the Nats air traffic control centre in Swanwick, operations have started up well at Heathrow on what is a very busy day.
"There will, though, be cancellations to some flights because aircraft and crew are out of position.
"Passengers due to depart today should check the status of their flight with their airline before travelling to Heathrow. We are very sorry for the disruption to passengers' journeys."
Gatwick reported that there had been 16 cancellations and seven diversions of inbound flights yesterday but said the airport was running normally today.
A British Airways spokeswoman said: "We've been working hard to look after our customers following the air traffic control system failure, yesterday.
"We don't expect to see any further significant disruption to our flights, for the remainder of the day."
Airports as far north as Aberdeen and Edinburgh were also affected by the computer problem. Other airports that reported delays yesterday afternoon included Manchester, Stansted and Luton.
Transport Secretary Patrick McLoughlin has described the disruption as unacceptable.
MP Louise Ellman, chair of the Transport Committee, said Mr McLoughlin will be asked about the incident when he appears before the panel on Monday.
She said: "I am very concerned about this major breakdown in UK air traffic control and the impact of the disruption this failure has caused on airports, airlines and passengers.
"In such circumstances safety remains paramount, but going forward it is vital that we establish what happened yesterday, and what NATS must do in order to ensure the same problem does not recur in future.
"I also hope that the committee will call NATS and the CAA to give evidence to us, so that we can get to the bottom of what led up to this weekend's events."
Airline easyJet said it cancelled ten flights to and from London Gatwick yesterday.
A spokesman said that easyJet "can confirm it has made no cancellations today following yesterday's failure of NATS' air traffic control system and will operate its full planned schedule of flights today".
He added: "We would like to thank passengers for their patience and apologise for any inconvenience."
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