The British Army almost brought down a troop-carrying RAF helicopter when one of its own drones flew within 60ft of the giant aircraft, a report found.
The £15million Merlin vehicle and metre-long Desert Hawk surveillance drone should have been flying in their own restricted areas.
But they came close to crashing during a massive military training exercise, as crews blamed map-reading errors and a dispute over airspace for the near catastrophe.
Britain’s Joint Helicopter command has been told to review its procedures after serious errors put lives at risk.
However there was no way of telling whose fault the incident was, experts from the UK Airprox Board, which investigates near misses in British airspace, concluded.
The incident happened above a replica Afghan village north of Thetford, Norfolk, which has been purpose built in the 30,000-acre Stanford training area.
The Desert Hawk, which was charged with protecting the perimeter of a replica base, was operated by a ground team from 1 Artillery Brigade, while the huge troop- and supply-carrying RAF Merlin was piloted across the exercise area at just 100ft.
All helicopter and jet pilots were warned to stay out of a four square kilometre area reserved for the drone, which flies up to 500ft above ground level, before the exercise began.
The Merlin crew claimed the near miss happened because the unmanned aerial vehicle (UAV) and the five personnel operating it were 300m outside their zone.
But the drone’s commander said it was in the process of landing at the correct pre-agreed spot when the Merlin suddenly appeared overhead.
The Airprox Board report concluded: “The UAV Operator was instructed not to abort the landing and to ‘let it come in hard into trees’ to avoid possible collision with the Merlin if the landing was aborted”.
However, investigators could not decide who was to blame as there was not enough information to show who was reading the maps correctly.
“In the absence of any recorded information, the Board were faced with a simple conflict in reports regarding the actual location of the UAV ground control station and the Merlin’s ground track, which members agreed was not possible to resolve conclusively with the information available,” the report found.
“The Board emphasised that Airprox include all forms of aerial vehicle, and that with the increasing use of UAV, Airprox occurrences involving them were bound to increase.
“With this in mind, the Board resolved to recommend that in order to ensure robust deconfliction from other airspace users, HQ JHC consider reviewing the robustness and coordination of Remotely Piloted Aerial Systems operations.”
The event was reported last July – the same month a civilian drone flew within 20ft of an Airbus A320 as it landed at Heathrow Airport.
A probe into the near-miss, or ‘airprox’, found the UAV almost hit the passenger jet just 700ft above the runway.