Thousands of air passengers in Britain faced travel chaos on Thursday after a computer fault at the main air traffic control centre caused delays and cancellations.

Some flights at London Heathrow, the world's busiest international airport, were cancelled or delayed as controllers restricted take-offs and landings around the capital while they operated with manual systems.

The problem was resolved by 7pm, according to air traffic control operators NATS who apologised for the delays, but insisted that safety "has not been compromised at any stage."

BAA, which runs London's Heathrow, Gatwick and Stansted, said earlier: "Aircraft continue to land and depart, however the process is slower than normal, which means that inevitably, some flights will be delayed and some will be cancelled."

A spokesperson added that at Heathrow, flights were leaving every two minutes where normally they would take off every 90 seconds.

London's Luton Airport was facing "quite severe delays" and eight flights had been cancelled, spokesperson Jo Lloyd told AFP. She added that between 35 and 40 would normally be operating Thursday evening.

London's City Airport was running four departures an hour compared to the normal rate of 30, spokesperson Rupa Haria told AFP.

The fault, affecting the London area only, occurred at the Air Traffic Control Centre in Hampshire, southern England, at around 4pm Thursday.

"Safety has not been compromised at any stage and we sincerely apologise to those who have been inconvenienced," said NATS director of operational performance, Ian Hall.

"We are working closely with airlines to increase capacity this evening to help minimise delays and impact on the rest of today's schedules."

He added that NATS handles nearly 2.5 million flights a year and "our systems are incredibly resilient," but they were not immune to difficulties.

AFP