Two jumbo jets had a near miss in Scottish airspace because the pilots failed to follow instructions to avoid a collision, a report has found.
The incident happened on June 23 as both crews followed each others' instructions instead.
At one point during the incident, above the North Sea around 100 miles off the north east coast, the two Boeing 747s were just 3.9 nautical miles apart horizontally and 100ft apart vertically.
Details of the incident were revealed in a report by the UK Airprox Board which examines near-misses in British airspace.
At the time of the incident, both planes were in airspace under the command of air traffic controllers in Prestwick, Ayrshire.
The report noted that one of the 747s requested to climb to an "oceanic level" height. It was allowed to do so but this resulted in a "potential confliction" with another 747 which was "on a converging track".
But the controllers, realising there was "the potential for conflict", took "timely action" by issuing appropriate avoiding instructions to both aircraft.
"Had the pilots complied with these instructions, simulation indicated that separation would not have been lost," the report found.
The Airprox Board concluded that the pilots "flew into conflict because, although they acknowledged timely avoiding action, they did not follow it".
The fact that the aeroplanes had been able to climb to the same level was identified as a contributory factor in the incident.
Any chance of collision was averted after the planes' automated Traffic Alert & Collision Avoidance systems kicked in.
The board was unable to come up with concrete reasons for why the crews did not follow the instructions.
The report stated: "It was apparent that both crews had taken each others' instructions, and the board found it hard to determine why this had occurred; unfortunately no human factor report was available from either crew.
"The board was surprised that all four pilots had misheard or misinterpreted the avoiding action instructions despite at least one of the crews reading them back correctly.
"It was possible that the crews may have been distracted because this would have been about the time that they would have been receiving their oceanic clearances on data-link.
"Another possibility mooted by an airline-pilot member was that, having settled into their trans-Atlantic routine, it was unusual for pilots to be issued with avoiding action instructions at that altitude and location.
"Expecting only routine information to be transmitted at that time, they may have been perplexed by the avoiding action information and instinctively responded without properly assimilating it."