Hundreds of flights across the country, including from London's main Heathrow airport, were delayed when a computer undergoing an upgrade went on the blink at around 6 am (0500 GMT), officials said.

That was just at the time when long-haul flights from Asia and North America were coming in to land at British airports, and when domestic and European flights were just starting to get into the air.

"Our computer system is now fully operational," said Adrian Yalland, a spokesman for National Air Traffic Services (NATS), after technicians fixed the glitch by 7 am.

The incident nevertheless called into question the reliability of Britain's semi-privatised air traffic control system, which under NATS has been undergoing a long-term modernisation programme.

BAA, which operates Heathrow, one of the world's busiest airports, advised travellers to check in as usual but warned them to expect delays throughout the day.

"There is an average of two-hour delays on departures and restrictions on arrivals," said Mark Pearson, a spokesman for Heathrow, which handles 1,250 flights a day.

British Airways said all its flights were "subject to disruption and cancellations."

As of mid-morning, it said, it had seven flights out of London Heathrow and three out of Gatwick, the capital's second airport.

An AFP reporter at Heathrow's Terminal Four found that, while long-haul flights to places like Chicago, Houston and Abu Dhabi were boarding as usual, there were long and growing queues for European destinations.

"The long hauls are fine. It's just the European flights we're having trouble with," a British Airways representative said.

Transport Secretary Alistair Darling said Thursday's breakdown involved computers at West Drayton, north of Heathrow, rather than those at a new 823 million pound (1.235 billion euro, 1.5 billion dollar) facility at Swanwick, near Southampton in southern England, which has been dogged by problems before.