National Air Traffic Services (NATS), the organization responsible for managing air traffic in the United Kingdom, has released its first detailed report about a system failure that caused widespread flight disruptions on September 8 and 9, 2026. The incident occurred when an automated process for assigning an aircraft identification code was interrupted briefly by a prioritized task. When the process resumed, a previously unknown software bug caused data errors, disrupting flight operations. Martin Rolfe, NATS's Chief Executive Officer, explained that the issue originated in a specific section of the flight data system and was traced back to a small part of the code. The outage began at the Swanwick control center in southern England on September 8. NATS detected the problem in its flight plan processing system and implemented a temporary technical fix by the afternoon. However, the effects continued into the next day, as airlines had to reposition aircraft and crews to manage the impact. Although NATS confirmed that safety was not compromised, the system's reduced capacity led to a chain reaction of flight cancellations and delays. According to Cirium, a leading aviation data provider, about 1,750 flights were canceled on September 8, with at least 291 more cancellations on September 9, totaling over 2,000 flights over two days. The Airlines UK association, which represents British airlines, reported that more than 330,000 passengers were affected by the disruptions. Approximately 155,000 had their flights canceled, and nearly 180,000 faced delays. London-Heathrow, the largest airport in the UK, was the most affected, with the majority of canceled flights originating there. Other affected airports included London-Gatwick, Liverpool, Birmingham, and Edinburgh, while Dublin Airport canceled several dozen flights as well. Airlines have called for accountability, as they had to adjust schedules, move aircraft and crews, and assist stranded passengers. According to Reuters, the financial costs to airlines were in the millions of pounds. Some carriers even demanded that Rolfe resign, but he declined, stating that mitigation measures were already in place while a permanent solution was being tested. The NATS CEO apologized and assured that temporary fixes were working until a long-term resolution was available. The UK air traffic control manager confirmed that the incident was not due to human error by either civilian or military operators and ruled out a cyberattack. UK Transport Minister Heidi Alexander described the disruptions as "totally unacceptable" and ordered an independent investigation by the Civil Aviation Authority (CAA). The minister emphasized the need to understand why the issue was not detected and corrected before causing chaos. The CAA must provide an update within six months, and the review will also assess whether NATS's systems are prepared to handle such failures. This incident follows several recent disruptions in UK air traffic control, but NATS clarified that the September 8 outage was not related to the 2023 incident that affected over 700,000 passengers or the radar failure in July 2025.