The incident was not serious because the two aircraft were never at risk of colliding - at their closest point they were two miles apart on a parallel course. But some controllers point out that the absence of any danger was in part because of good luck on the part of all parties.
A Boeing 777 from Rio De Janeiro was at 34,000ft and a Boeing 737 from Valencia was at 32,000ft, both flying in a similar direction and with a lateral separation of about two miles.
At this point the aircraft positions were fully compliant with international aviation law because they were separated by 2,000ft. The rules dictate that aircraft must be separated by a height difference of at least 1,000ft or by at least three miles if travelling side by side, said National Air Traffic Services, which runs the Swanwick centre.
When a trainee controller asked the Boeing 777 to descend to 26,000ft, crossing the height level of the 737 below, this was officially a near-miss because, although laterally the two aircraft were two miles apart, this was less than the required three miles.
The instructor, sitting beside the trainee, had a back-up system which in principle is like the dual controls on a driving school car. Normally the instructor would have used the switch on his override communication system to countermand the trainee's instruction by speaking directly to the pilots. But his override equipment failed.
When the trainee realised his mistake he directed the 737 to turn to one side, increasing the separation between the aircraft.
Some controllers say that the failure of the override system could have been more serious if, at the time of the mistake, one aircraft had been directly above the other and had been wrongly asked to descend.