Last Monday's air traffic control fault in Britain was a result of circumstances that had never happened before, UK's air traffic control service said Wednesday.
More than 1,500 flights departing from UK airports were cancelled and hundreds more were delayed after the country's air traffic control system was hit by technical fault on the August bank holiday.
In a statement, the National Air Traffic Services (NATS) said the problem had been identified with a flight plan processing sub-system called Flight Plan Reception Suite Automated-Replacement (FPRSA-R).
"A small but important part of NATS' overall air traffic control technical infrastructure, it was found to have encountered an extremely rare set of circumstances presented by a flight plan that included two identically named, but separate waypoint markers outside of UK airspace," it said.
The statement added that this led to a "critical exception" whereby both the primary system and its backup "entered a fail-safe mode."
The NATS stressed it has taken required steps to ensure the incident cannot be repeated.
"This scenario had never been encountered before, with the system having previously processed more than 15 million flight plans over the 5 years it has been in service," it added.
Reiterating his apology over the incident, Martin Rolfe, CEO of NATS, said they have taken the required steps to ensure the "extremely rare" incident is never repeated.
Transport Secretary Mark Harper said he welcomed NATS' preliminary report and announcement by the Civil Aviation Authority of an independent review into the incident.
"Thousands of passengers faced disruption as a result of the failure, with over 1500 flights cancelled and hundreds more delayed," he said in a statement, adding he will chair a further meeting on the issue.
