The UK's air traffic control operator National Air Traffic Services (Nats) has identified a "software defect" in legacy systems as the cause of last week's outage that grounded over 2,000 flights and delayed hundreds of thousands of passengers. The defect corrupted an instruction sent to a single aircraft, causing the system to pause when processing simultaneous requests at the same millisecond, ultimately forcing Nats to take its entire system offline for several hours.

Nats is under intense pressure from airlines and the government following the chaos. Transport Secretary Heidi Alexander called the incident "completely unacceptable" and announced a six-month investigation by the Civil Aviation Authority to examine the agency's investments and underlying causes. Nats Chief Executive Martin Rolfe apologised for the disruption while stressing safety was never compromised.

The Ministry of Defence stated it did not believe it was at fault, without confirming involvement. Nats reported it has invested over one billion pounds in systems and technology over the past decade and will submit plans to invest an additional billion pounds by the end of 2033.