A previously unknown software defect in the UK’s air traffic management system caused the 8 September 2026 outage that led to more than 2,000 flight cancellations and widespread delays, according to a preliminary report from NATS.

The UK air traffic services provider published its initial findings on 18 September, saying the problem originated in a small part of the National Airspace System’s flight-data software.

What caused the outage?

NATS says the defect was triggered when a high-priority request interrupted the normal processing of an aircraft identification-code request. That sequence corrupted flight data and caused protective backup procedures to activate.

To maintain safe operations, NATS reduced the number of flights that could be processed through affected parts of UK airspace. The resulting restrictions lasted for several hours and created knock-on disruption across major airports.

More than 2,000 flights were cancelled

Reuters reported that more than 2,000 flights were cancelled because of the disruption, with hundreds of thousands of passengers affected by cancellations, delays and aircraft being out of position.

The scale of the impact reflected how tightly interconnected airport schedules are. Once aircraft and crews are displaced, disruption can continue even after the original technical problem has been resolved.

NATS says safety was not compromised

NATS chief executive Martin Rolfe said the issue was a software problem rather than an error by military or civilian operators. The company also said the incident was unrelated to the major UK air traffic disruption in August 2023.

NATS said safety was not compromised because the system’s protective processes worked as designed and traffic restrictions were introduced when required.

Temporary mitigation in place

The operator says engineers have identified the affected code and put temporary mitigation measures in place. A permanent software fix is being safety-tested before deployment.

Because air traffic systems are safety-critical, changes generally require extensive validation before they can be introduced into live operations.

Independent review requested

UK Transport Secretary Heidi Alexander requested an independent review by the Civil Aviation Authority to examine the incident and whether further action is needed to improve system resilience.

The review is expected to consider not only the technical fault but also the operational response, contingency planning and the broader reliability of critical air traffic infrastructure.

What travellers should know

The outage itself has been resolved, but passengers affected by major delays or cancellations should check directly with their airline regarding rebooking, refunds and any rights that may apply to their journey.

Air traffic control failures can fall outside an airline’s direct control, so compensation rules may differ from cases where disruption is caused by the carrier itself. Travellers should keep booking confirmations, receipts and communication from the airline if they intend to make a claim.

Why the findings matter

The report gives the aviation industry a clearer explanation for one of the UK’s most disruptive air traffic incidents in recent years. It also shifts attention to whether software architecture, testing and resilience controls can be strengthened enough to prevent a similar fault from causing another large-scale network disruption.

NATS says a fuller report will follow after further investigation.