The computer problem that grounded thousands of passengers last week was caused by a single workstation, National Air Traffic Services (Nats) has revealed.
The problem caused chaos as flights were delayed at airports across the UK on Friday 17 May.
A Nats spokesman said the problem had been traced to a single workstation out of the 200 at the new air traffic control centre in Swanwick, Hampshire.
The problems followed a routine software upgrade. "Overnight we upgraded part of the software, which is a regular monthly event," the spokesman said. "When the morning shift arrived they couldn't open up all the workstations to offer full capacity"
At night the workstations are linked together so that a small team of controllers can cover more than one sector of airspace. They are separated again in the morning so that the centre can run at full capacity.
But a problem with one workstation meant that none of them could be unlocked from each other, reducing capacity.
The centre was left running at 50 per cent capacity, but this was up to 75 per cent by mid-morning and 100 per cent by 11.30am.
The workstation has now been withdrawn from service.
Nats said an engineering report into what went wrong is expected very soon. "The engineers have been looking at precisely what happened. We will be taking steps to make sure that this is not repeated. But safety was not compromised," said the spokesman.
The monthly software upgrades are to continue. "They must go on because they reflect changes in the air traffic environment," the spokesman said."It's difficult to put a backup in place for every [problem]. But the process we follow is such that we can handle the aircraft safely," he added.
The Swanwick centre, which went live in January five years late and £180m over budget, has already had to be upgraded several times.
An earlier upgrade in March delayed flights because restrictions were put on the number of flights while it took place.
And last month, problems with the Flight Data Processing System (FDPS) at the West Drayton air traffic control centre delayed more passengers.
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