A business jet narrowly missed colliding in mid-air with a Turkish Airlines' Boeing 777 with 232 passengers aboard.
The incident happened in July over London shortly after the jet, a Citation 525, took off from London City Airport.
The UK Air Accidents Investigation Branch (AAIB) described the incident as "serious" in its report published today (September 9).
The Boeing, which was on its way to London Heathrow, was flying at about 4,000 feet when the jet, carrying three people, came within about half a mile of it.
The jet, which was flying about 100-200 feet below the airline, had been cleared by air traffic controllers to fly to 3,000 feet.
The AAIB report said the two-man jet crew "read back their cleared altitude as 4,000 ft, an error that was not noticed by the Tower controller".
It said that at the same time, the Boeing was cleared to descend to 4,000 feet prior to landing at Heathrow.
The report said the jet crew saw the Boeing in time to take "effective avoiding action."
www.aaib.gov.uk