Two Boeing 747s carrying around 1,000 passengers almost collided in mid air over Scotland because pilots confused their flight instructions.
The aircraft were 30 miles to the north of Glasgow and preparing to fly across the Atlantic when an air traffic controller spotted on a radar sweep that they were moving closer together.
The controller immediately ordered the aircraft on the left to make a left turn and the one on the right to make a right turn, to move them further apart.
But the two pilots on board each aircraft did just the opposite, bringing their aircraft closer together.
When the danger was first spotted the aircraft were 10 miles apart, but within a minute they were less than three miles away from each other and still closing in.
A crash was only averted when the pilots physically saw the other aircraft, with one climbing and the other diving. At their closest, one aircraft was just 100ft above the other.
Details of the incident, which occurred on June 23, came to light following a report by the UK Airprox Board, which examines near-misses in British airspace, the Daily Telegraph reported. The airlines involved were not named.
"It was apparent that both crews had taken each others' instructions,” the authors of the report said, but experts "found it hard to determine why this had occurred".
It read: "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.
"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," it said.
The Board said that the danger was needlessly increased because the aircraft had earlier been ordered by air traffic control to fly at the same height of 34,000ft.
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