A software defect was responsible for the recent UK air traffic control outage that resulted in the cancellation of about 2,000 flights and widespread disruption, the head of NATS, the system operator, said on Friday. The incident affected hundreds of thousands of passengers and led to significant operational and financial challenges for airlines.

Martin Rolfe, CEO of NATS, detailed in a preliminary report that the fault originated from a small section of code within the flight data system. The problem occurred when a request for an identification code from an aircraft was interrupted by a separate, higher-priority request, causing the initial process to pause. The report indicated that if the higher-priority request had come just one millisecond earlier or later, the system would have functioned normally. This timing error triggered backup procedures requiring manual flight handovers, which reduced system capacity and led to cascading delays.

Rolfe emphasized that safety was never compromised during the outage and rejected suggestions that the disruption was related to the previous 2023 incident or any military activity, as had been speculated in some media reports. He also confirmed that a mitigation plan is currently in place and that a permanent fix is undergoing safety testing before full deployment.

The disruption reignited criticism from airlines, with Ryanair restating calls for Rolfe’s resignation. Neil McMahon, Ryanair’s Chief Operations Officer, dismissed the explanation as inadequate and urged for leadership changes to prevent future failures. The company has repeatedly called for Rolfe’s removal following the 2023 air traffic control failure that reportedly cost airlines around £100 million ($136 million).

Transport Minister Heidi Alexander described the outage as “completely unacceptable” and demanded a swift and thorough understanding of why the software issue had gone undetected. She has ordered an independent review by the Civil Aviation Authority to assess the resilience of the UK's air traffic infrastructure and ensure it meets future demands.

This latest incident adds to a series of operational challenges faced by NATS in recent years, including a radar fault that caused 150 cancellations in July 2025. Despite the setbacks, NATS reports that once the problem was identified, it was resolved under controlled conditions while maintaining a limited air traffic service throughout the outage.