A software defect in the system used to manage UK airspace triggered restrictions that disrupted hundreds of thousands of passengers and led to more than 2,000 flight cancellations, according to National Air Traffic Services, known as Nats.
The failure began at 10:00am on 8 September, when a defect produced corrupted data during an event lasting a millisecond. Nats said the fault was in a small subsection of code within its National Airspace System module, which handles the allocation of squawk codes. Controllers use those codes to identify aircraft on radar.
The operator said the problem occurred when the system received a message relating to a higher-priority activity while it was processing a manual request for a squawk code allocation. The disruption affected flights arriving at and leaving the UK’s main airports, including Heathrow, Gatwick, Luton, Glasgow, Stansted, Manchester and Birmingham.
It took another two and a half hours for the seriousness of the problem to become apparent. Nats first confirmed that it was investigating a technical issue at 13:46 BST on Tuesday. Around 90 minutes later, it identified the flight processing system as the source of the issue.
A system restart began just after 3:15pm. Nats said by 19:30 that the technical issue had been resolved, although restrictions on UK airspace were lifted only at 7:30pm. The restrictions lasted approximately four to six hours.
Passengers were left for hours in terminals and on airport runways. The effects continued after the system was restored. The backlog of disrupted passengers was cleared more than two days later, while flight schedules stabilised by Thursday evening.
Cirium, an aviation analytics firm, estimated that at least 300,000 passengers were affected, based on a 30 per cent cancellation rate of flights. Nats said its report had found no evidence at this stage that a malicious actor or cyber-related activity caused the incident.
It is unclear why the issue was not detected before 8 September, why its scale took two and a half hours to emerge, or when a permanent repair will be deployed. Nats said mitigation measures are in place while it tests and deploys a lasting fix.
The episode is the third national air traffic control incident in three years. A computer shutdown in 2023 affected 700,000 passengers. A Nats outage in 2025 resulted in 150 cancellations. Nats said the latest disruption was unrelated to the 2023 incident or to a radar issue in July of the previous year.
Heidi Alexander, the Transport Secretary, described the disruption as “completely unacceptable” and said the technical failure had been avoidable. She ordered Nats to complete a full investigation within two months.
Alexander also tasked the Civil Aviation Authority with an independent review of Nats’ findings and of its investment plans to improve resilience. The regulator is due to provide an update in six months. The review will examine an operator that runs UK air traffic control systems from its Swanwick Centre in Hampshire and is 49% owned by the government.
Nats has apologised for its third major technical fault in three years. Its chief executive, Martin Rolfe, who has led the organisation for 11 years, said he would not resign. Ryanair chief executive Michael O’Leary has called for Rolfe to be sacked.
Airlines UK is writing to Nats to seek immediate compensation for costs arising from the disruption. The full cost to airlines and the wider aviation industry has yet to be established. The Civil Aviation Authority said passengers were unlikely to qualify for compensation because the event was likely to be treated as “extraordinary circumstances”.
The investigations will determine whether Nats’ systems and investment plans require further changes. They will also test whether government oversight of a partly state-owned operator will be strengthened after repeated failures in a service central to the movement of flights through UK airspace.
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