Air experts are investigating how two jets came close to collision after an equipment failure at a new £623m air traffic control centre.
On Monday (17 June), a trainee controller at the centre mistakenly put two Heathrow-bound British Airways planes on a possible collision course. His instructor pressed an over-ride button to warn the pilots, but the device failed.
A warning did get through to one of the aircraft, but incident is now being officially investigated as a near miss.
The Swanwick system has been blighted with problems since it went live in January.
The Health and Safety Executive is investigating complaints by air traffic controllers that they have difficulty reading computer screens and have repeatedly misread the height of aircraft by thousands of feet and sent a plane into the wrong airspace sector.
In May, problems with a routine overnight software upgrade left controllers staring at blank screens and led to long delays and flight cancellations at most British airports. This followed three failures in air traffic systems in the previous two months.
Earlier this month CW360.com reported that internal documents from National Air Traffic Control Services, showed the number of controllers filing reports complaining of excessive workloads has doubled this year.