'Millisecond' software error caused air traffic outage that grounded thousands of flights

The air traffic outage that grounded thousands of flights was not caused by a 'military operation' but by a software problem, an initial investigation has found.

NATS, the UK's major provider of air traffic services, said the software defect led to corrupted information which affected flight data updates, requiring restrictions - it added that the events "happened in the space of a millisecond".

Despite the issue only affecting flights in the London Area Control centre, flight restrictions had to be put in place across the UK so that the National Airspace System (NAS) - which underpins UK airspace management - could be restarted.

An investigation had ruled out a cyber attack by a hostile actor or sabotage, National Air Traffic Services (NATS) said.

The glitch, which led to 2,000 flight cancellations and disrupted hundreds of thousands of journeys, raised concerns about NATS' systems and led some to call for the resignation of Martin Rolfe, chief executive officer of NATS.

Mr Rolfe said on Friday: "This was a software issue in a specific part of our flight data system, that we have traced to a small subsection of coding.

"The issue has been identified and mitigation is in place while a permanent fix is safety tested and deployed."

He also said the shutdown was not related to a problem in 2023 which also caused widespread disruption.

How a paused code request triggered two days of chaos

The preliminary report, which was requested by the Transport Secretary Heidi Alexander, explains how a manual request for an aircraft code that was being processed was paused after the NAS system received a message for a higher priority activity.

When processing of the aircraft code request resumed, the software defect meant it did not resume correctly, triggering the chain reaction that led to the grounding of thousands of flights.

The investigation shows that while the initial error happened at 10am on 8 September, it wasn't apparent until two and a half hours later that a major incident was underway.

While restrictions were only in place for six hours, and NATS operations returned to normal the same evening, it took more than two days for the backlog of passenger disruption to be cleared.

The transport secretary has also ordered an independent review by the UK Civil Aviation Authority (CAA) and said the disruption was "completely unacceptable".

More from Sky News:
All government departments to refresh war plans
20 women had unnecessary mastectomies, says NHS trust

The incident followed a shutdown in August 2023 ​which also caused travel chaos, costing airlines £100m.

A smaller technical ​issue which NATS was behind in July 2025 also caused 150 ⁠cancellations.

Ryanair, Europe's biggest airline, has repeatedly called for Mr Rolfe to resign since the 2023 failure.

The NATS boss said: "I would like to apologise again, very sincerely, to everyone who was affected last week. It's our job to get people where they want to go, quickly and without delay and we are devastated when that goes wrong."

However, he insisted that his primary role was to keep "our skies safe" and that "at no point last week was safety in question".

NATS added that it had invested over £1bn in systems and technology over the past decade and plans to submits plans to the UK CAA to invest another £1bn before the end of 2033.

(c) Sky News 2026: 'Millisecond' software error caused air traffic outage that grounded thousands of flights