Long lines ... People queue in the luggage hall of Terminal 5 at Heathrow Airport as London's airspace was closed due to a computer failure at one of Britain's two air traffic control centres. Picture: AP Photo/Matt Dunham Source: AP

“There are 38 flights cancelled today as a result of (the computer fault),” said a Heathrow spokesman, adding that 1300 flights were scheduled to and from the airport that day.

Departures were blocked for more than an hour on Friday and arrivals diverted when NATS, Britain’s main air navigation service, restricted traffic volumes because of a problem that has “not been seen before”.

Waiting ... People stuck in the luggage hall of Terminal 5 at Heathrow Airport in London due to a computer failure at one of Britain's two air traffic control centres. Picture: AP Photo/Matt Dunham Source: AP

The system failure occurred at the state-of-the-art STG700 million ($A1.28 billion) Swanwick control centre near Portsmouth on the southern English coast.

In a statement, NATS said the problem occurred when trying to switch work stations between the “standby” and “on” states.

“In this instance a transition between the two states caused a failure in the system which has not been seen before,” it said.

“The failure meant that the controllers were unable to access all of the data regarding individual flight plans which significantly increases their workload.

Killing time ... A man rests on seats in the departures lounge in the north terminal at Gatwick Airport, south of London after a computer failure at an air traffic control centre sparked travel chaos in Britain as the peak Christmas season gets underway. Picture: AFP PHOTO / JUSTIN TALLIS Source: AFP

“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,” it added.

Despite having full radar capabilities, NATS “immediately took steps to reduce the traffic into and out of the UK network” in order to “maintain a safe operation for the flying public.”

The London airports system, grouping Heathrow with Gatwick, Stansted and Luton, is the busiest hub in the world with around 135 million passengers a year.

London’s Heathrow alone is Europe’s busiest in terms of passenger numbers, and the world’s busiest for international passenger traffic.

Transport minister Patrick McLoughlin called the disruption “simply unacceptable” and said he had asked NATS for a full explanation.

“I also want to know what steps will be taken to prevent this happening again,” he added.