Air traffic control officials have revealed details of what caused widespread disruption that affected thousands of passengers stuck at airports.
The National Air Traffic Services (NATS) previously said an initial investigation found the problems were caused by flight data which its system "didn't understand" and "couldn't interpret".
And the organisation said, in a preliminary report released today, that it had confirmed the root cause of the issues - while repeating its apology and insisting safety was not compromised at any point.
NATS chief executive Martin Rolfe said the technical glitch which caused the widespread disruption was a "one in 15 million" occurrence.
A statement issued by NATS said a problem had been identified with a flight plan processing 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.
"This led to a 'critical exception' whereby both the primary system and its backup entered a fail-safe mode.
"In these circumstances, the system could not reject the flight plan without a clear understanding of what possible impact it may have had.
"Nor could it be allowed through and risk presenting air traffic controllers with incorrect safety critical information. On the day, the time taken to recover was driven by the need to identify the problem and the specific data, isolate and remove it in a controlled way, and then test it to ensure it could be returned safely into operation."
It added that the scenario had never been encountered before across more than 15 million flight plans over the five years it has been in service.
"Steps have been taken to ensure the incident cannot be repeated," the statement said.
Mr Rolfe said: "Keeping the sky safe is what guides every action we take, and that was our priority during last week's incident.
"I would like to reiterate my apology for the effects it had on so many people, including our airline and airport customers. Incidents like this are extremely rare and we have put measures in place to ensure it does not happen again."
The failure is estimated to have cost airlines £100m, according to the head of the International Air Transport Association (IATA), Willie Walsh.
More than 1,000 flights departing UK airports were cancelled due to the problems last week, data from aviation analytics company Cirium revealed.
Transport Secretary Mark Harper said: "Thousands of passengers faced disruption as a result of the failure, with over 1,500 flights cancelled and hundreds more delayed. I once again want to echo NATS's apology to those who were caught up in it, with a technical fix now identified to ensure that such an incident does not recur."
He added he would be holding further talks tomorrow with industry officials and NATS - and the Civil Aviation Authority (CAA) which has announced an independent review into the event.