Heathrow Airport saw fresh delays and cancellations Saturday as it recovered from an unprecedented computer glitch that wreaked havoc with the London airport system, the world's busiest hub.
The system outage at the headquarters of Britain's main air traffic controller NATS delayed departures, diverted arrivals and led to the cancellations of dozens of flights Friday evening.
London Heathrow, one of the world's busiest airports, cancelled 38 flights and saw a string of delays early Saturday. Services at London Gatwick, Stansted and Luton were operating as normal.
NATS said Saturday that the shutdown was linked to a problem caused when the computers used by air traffic controllers switched from being on standby to in use.
"In this instance a transition between the two states caused a failure in the system which has not been seen before," it said in a statement.
"The failure meant that the controllers were unable to access all of the data regarding individual flight plans, which significantly increases their workload."
NATS insisted that "at no time was safety compromised in any way."
The system failure occurred at the state-of-the-art Swanwick control centre near Portsmouth on the southern English coast.
Swanwick has a history of IT problems. It was initially scheduled to open in 1997 at a cost of £132 million but it did not start work until 2002 at a cost of £700 million (883 million euros, $1.1 billion) due to software issues.
Last December, another computer problem took 12 hours to fix and caused significant flight disruptions.
NATS is a joint venture between the government and the private sector.
The outage forced some passengers planning pre-Christmas getaways to abandon their trips altogether.
Bertie Rowell told the BBC he had been due to fly to Paris from Heathrow Friday but his flight had been cancelled.
"We had to stand for two-and-a-half hours in a queue with no explanation of what was happening," he said.
"We stayed in a hotel hoping to go on standby this morning. At 6am we went back to the airport only to be told we were better off going home."
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.
Transport Secretary Patrick McLoughlin, from Prime Minister David Cameron's Conservative party, called the disruption "simply unacceptable" and said he had asked NATS for a full explanation.
McLoughlin faces questions on the issue in the House of Commons Monday during an appearance before its Transport Select Committee.