A business jet had to take avoiding action to prevent a potential mid-air collision with a Heathrow-bound airliner with 232 passengers aboard, an accident report has revealed.
The German-owned Citation 525 jet passed only just over half a mile away and 100ft to 200ft below the Turkish Airlines' Boeing 777 plane in the skies over London, the Air Accidents Investigation Branch (AAIB) report said.
The Citation had been cleared by the control tower at London City Airport to climb, initially to 3,000ft.
But acknowledging the instruction, the crew said it would be climbing to 4,000ft - a "readback" mistake not noticed by the London City tower controller.
The Boeing 777 had been cleared to descend to 4,000ft and it was at this height that it passed the Citation, which had two crew and one passenger aboard.
The AAIB report, which described the events of the afternoon of July 27 as a "serious incident", said the Boeing 777 had not "followed the commands" from three on-board collision-avoidance warnings.
The report added that the only person to see the Citation was a pilot occupying the right observer seat who saw it "pass west of them at an estimated 100 to 200ft below".
The AAIB said the Citation captain had later filed an airprox (aircraft proximity) report in which he stated the crew had been given clearance to climb to 4,000ft.
He said he had the Boeing 777 in sight "all the time" and initially thought his aircraft would be "well above" the Boeing as he crossed its track.
The AAB said that if the weather had been bad the Citation would not have been able to see the Boeing and would therefore not have been able to take effective avoiding action.