The head of British air traffic control on Wednesday blamed a "one in 15 million" event that caused the country's worst systems failure in almost 10 years, stranding thousands of passengers.
Air traffic control systems were temporarily hit after the incident on August 28, causing hundreds of delays or cancellations to flights departing and arriving in the UK and costing carriers an estimated £100 million ($125 million).
Martin Rolfe, chief executive of National Air Traffic Services (NATS), blamed the outage on duplicate "markers" on a flight plan that the system could not process.
He told the BBC it was "a one in 15 million" chance, and meant engineers had not encountered the problem before.
In its initial report published on Wednesday, NATS said that it received details of a flight that was due to cross UK airspace on August 28.
Two markers along the planned route had the same name but were in different places, crashing the system and triggering it, and the backup system, to automatically shut down for safety reasons.
"With planes and crew out of position and most flights already booked up, many people found themselves stuck abroad on what is usually a big day for travel -- a bank holiday -- facing long waits to get home," said Rolfe.
The last Monday of August -- a public holiday in England, Wales and Northern Ireland -- traditionally sees large numbers of passengers returning from summer vacations.
Transport Secretary Mark Harper said last week there would be an independent review into the worst incident of its kind for "nearly a decade".
London's main airports were the worst hit, with Heathrow and Gatwick cancelling dozens of flights Tuesday.
British Airways was the worst affected airline.
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