A computer failure at a key air traffic control center in Britain grounded many of the country's flights and stranded thousands of passengers on some of the world's busiest air routes. The system took about two hours to repair, but airports said the backlog of flights would cause more delays throughout the day.

A glitch in a new and supposedly improved computer software program that was to have logged the basics details of flights coming in and out of Britain was blamed for the problem. Air traffic control officials said that for safety reasons, they suspended all outgoing flights until the computers could be fixed.

The problem struck just as early intercontinental flights were beginning to arrive at British airports and scores of jetliners were being boarded for take-off.

The effects were being felt around the world as hundreds of flights were delayed both landing and taking off from some of aviation's busiest airports, including London's Heathrow.

Adrian Yalland from Britain's National Air Traffic Services says the computer problem was in a London center.

"The flight data processing system was being tested for a future upgrade overnight," explained Adrian Yalland. "The system was found to be unstable and purely as a safety precaution, the backup operation was activated."

For approximately an hour, planes were kept on the tarmac, while air traffic controllers focused on guiding in the inbound flights. It took another hour to get the system back in running order.

Mr. Yalland says this had the effect of creating a departure backlog but that safety was the overriding concern.

"We decided we would restrict the number of aircraft in the air at the time by restricting the number of take-offs that there were, or departures from airports, in order for us to be able to dedicate our resources to making sure that those aircraft that were already in the air were handled safely, that they were able to land safely," he said.

Four years ago, the air traffic control system experienced a similar problem.

By midday Thursday, average flights were running around two hours behind schedule in Britain, although that varies from airport to airport.

Air traffic controllers said they expected scheduled flights should be back to normal by the end of the day.