A Cambridge air ambulance helicopter came within half a second of a lethal collision with a drone.
The remote-controlled drone flew within a few feet of the East Anglian Air Ambulance and only missed colliding with it "by chance", according to a report seen by the Mail on Sunday.
The potentially deadly near miss happened while the helicopter was flying at 1,900ft on August 26, 2016.
Crewmembers were flying back to base in Cambridge from North London when passengers seated at the back of the aircraft spotted the four-bladed quadcopter drone.
It passed very close to the right side of the cabin and was close enough for them to see that it had four blades, was dark in colour, and had two red lights.
The pilot assessed the risk of collision as ‘high’ and alerted the authorities. Efforts to trace whoever was operating the drone were unsuccessful.
Prince William was not on shift at the time and was not piloting the aircraft in question.
He is due to leave his job with the Air Ambulance Service in the summer to fulfill more royal duties, after joining the service as a pilot in 2015.
Experts are becoming increasingly concerned that the proliferation of drones in Britain's skies could lead to a crash.
Reports to police about drones have risen by 352 per cent in a year.
There are also fears drones are being used by enterprising criminals in a variety of purposes.
It is believed burglars are using them to scope properties and thieves to identify pin numbers.
A spokeswoman for East Anglian Air Ambulance said: "There are strict rules that drone operators must follow and it is important they are aware of their responsibilities for safe operations at all times.
"The CAA and the Department for Transport have launched a safety awareness campaign which includes a new www.dronesafe.uk website, a #400ftBritain drone photography competition that highlights safety rules, and a free new app ‘Drone Assist' by NATS, which shows users where they should and should not fly in their locality."
