LONDON — An unprecedented computer systems failure touched off the recent mayhem that caused delays and canceled flights for thousands of passengers, the U.K.-based air traffic management company said Saturday.

In its first detailed explanation of the computer glitch that led to flight disruptions Friday, NATS said the problems at its control center in Swanwick occurred as more workstations were being brought on line to deal with an increase in traffic.

"In normal operations the number of workstations in use versus in standby fluctuates with the demands of the traffic being controlled," NATS said in a statement. "In this instance a transition between the two states caused a failure in the system which has not been seen before."

The computer failure made it impossible for the controllers to access data regarding individual flight plans. Planes were prevented from taking off, but those in the air and close to airports were allowed to land during the shutdown that lasted about 35 minutes.

Britain's government reacted with fury over the meltdown ahead of the holiday season, when travel demand is particularly high. Ministers are demanding a thorough explanation.

Computer software experts have said the problem appears to lie in the age of the systems — some of which date to the 1960s. The Swanwick center has been troubled since it opened 2002 — six years late and at twice its anticipated cost.

Problems over the past two days were particularly acute at London's Heathrow Airport, where flights were canceled both Friday and Saturday.

Though the shutdown was brief, Heathrow operates at nearly full capacity. So when the flights were canceled, there was no place in the schedule to slot them.