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It turns out that more than 1,000 flights were grounded after repeated air traffic control problems disrupted travel.
Budget carrier Ryanair confirmed the cancellations for September 2026, blaming two UK air traffic failures within a fortnight.
Disruption was compounded by staff strikes among Belgian air traffic controllers and volcanic ash clouds from Mount Etna.
Despite the grounded services, Ryanair carried 20.1 million passengers across more than 111,700 flights during the month, up 4 per cent on the previous year.
Rival carrier Wizz Air reported a 24 per cent rise to 7.78 million passengers over the same period.
However, Wizz Air confirmed plans to trim late-year capacity by 5 per cent due to jet fuel prices and Middle East conflict pressures.
Air traffic chaos
The airline figures follow severe disruption on September 8 when a National Air Traffic Services system failure brought flights to a halt.
More than 1,300 flights were grounded across Heathrow, Gatwick, Manchester and Stansted airports on that day alone.
Aviation analytics firm Cirium and tracking service Flightradar24 recorded more than 2,000 flights delayed or cancelled across a two-day period.
Ryanair chief operating officer Neal McMahon called for the resignation of NATS chief executive Martin Rolfe, labelling him an "overpaid failure".
Mr McMahon said NATS attributed the September 8 outage to a "spurious flight plan".
He noted the issue mirrored a breakdown in August 2023 that stranded roughly 700,000 passengers when a single French flight plan caused system collapse.
Repeated failures
Mr Rolfe issued an apology on BBC Radio 4's Today Programme following the September 8 collapse.
He insisted safety was never compromised and ruled out any form of cyber attack.
"Things do go wrong in complex systems," Mr Rolfe said during the broadcast.
Transport Secretary Heidi Alexander summoned NATS chairman Sir Warren East for urgent talks to address the repeated infrastructure failures.
Frustration escalated further when a separate connectivity issue struck the NATS Scottish control centre at Prestwick later in the month.
The fresh glitch forced another wave of flight cancellations across British airspace.
Ministers demanded assurances that air traffic systems are resilient enough to prevent future shutdowns from stranding travellers.