A drone came dangerously close to the path of a passenger plane after being flown illegally, an investigation has found.

The drone, which was flown above the 400ft limit, was deemed by the pilot to have caused a high risk of a crash with a Bombardier DHC-8 plane which had just left Southend Airport.

The pilot spotted the drone and described it as a “black drone with two red rotors” according to a report by the Airprox Board - the air safety watchdog who carried out the investigation.

The drone came within 50metres of the London-bound airliner on December 26 as it was higher than “cloud tops”.

A report published by Airprox Board said: “The board considered the pilot’s overall account of the incident portrayed a situation where safety had been much reduced below the norm to the extent that safety had not been assured.”

It is not known if any passengers were on board at the time. The incident was reported to Essex Police.

Sgt Aaron Connolly, of the Specialist Operations Department, said: “Anyone who flies an unmanned aerial vehicle (UAV) needs to be aware of their responsibilities to operate the vehicle within the law. These regulations are put in place to ensure no other persons are put at risk from the use of UAVs.”

A second lower risk incident was also investigated after a drone was spotted on the right-hand side of a plane near Southend Airport.

Again, this drone was being flown above the 400ft limit four days later on December 30 but was much further away. The pilot of the Airbus A319 reported seeing a “twinkling” red light and a “flashing blue light” about 1.85km away.

The Airprox Board said safety “had been reduced” there had been no risk of collision.

Southend Airport is working with manufacturers of drone detection systems in a bid to track radio signals from drones so they can be identified quickly.