An East Anglian Air Ambulance regularly piloted by the Duke of Cambridge "narrowly avoided" a collision with a remote-controlled drone, according to an official report.
The helicopter, which the royal was not in at the time, avoided a potentially deadly mid-air crash with the remote-controlled machine by "chance", 1,900ft above north London last August.
Investigators found there had been a "high" risk of the two aircraft coming into contact, "endangering the EC145 and its crew" over a busy area.
A report into the near-miss by the UK Airprox Board, which probes such incidents to improve air safety, suggested the drone was flying above its permitted altitude.
The Eurocopter 145, which is used by the East Anglian Air Ambulance (EAAA), has been flown repeatedly by William during his work for the service.
The drone pilot was never found, the review said.
In January, William said he would be leaving his role as a pilot to spend more time in London, where his children will be schooled.