Air traffic control company Nats has offered its first detailed explanation of the problem at its national centre in Swanwick, Hampshire.

More than 300 flights were cancelled or delayed after the computer failure.

A statement from Nats 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.

Flights grounded

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.

By Saturday morning, timetables were returning to normal, although Heathrow said 38 flights had been cancelled before 9.30am and Gatwick Airport was cancelling a handful of flights.

Friday's problems came just over a year after hundreds of flights were affected when problems arose with a telephone system at Nats in early December 2013.

There were reports passengers on some flights were unable to collect their luggage and were told that it would be sent on to them by courier.

Other airports where travellers suffered delays on Friday included Manchester, Birmingham, Stansted and Luton, but airports as far north as Aberdeen and Edinburgh were affected.

Transport Secretary Patrick McLoughlin said the disruption was unacceptable. He has been summoned before the Commons' Transport Select Committee on Monday to answer questions about the failure.