A drone came within 25 metres of a plane landing at Stansted Airport and posed a “serious risk of collision” while two planes had a near miss, reports have revealed.

The black and white four-rotor drone crossed left to right across the path of a Boeing 737 at 3,000ft on its final approach to the airport in May.

The pilots estimated the drone was about 25-50m away, but reported avoiding action was not necessary because it would not impact the aircraft.

However the UK Airprox Board, which assesses incidents in UK airspace, gave the near-miss a Category A risk rating and stated that “separation had been reduced to the bare minimum”.

Drone flight above 400ft is prohibited in airspace without air traffic control permission, and flying at levels of 3,000ft operators must have an additional observer under regulations

The board said even if an observer was being used, they would not be able to see the drone clearly at that level.

The report said: “Although the pilot had stated avoiding action had not been necessary because it was assessed the drone would not actually impact the aircraft, the board considered that separation had been reduced to the bare minimum.

“They therefore determined the risk to be Category A, a serious risk of collision had existed.”

The incident took place over Castle Camps in Cambridgeshire, and the drone operator was not traced.

The near-miss follows two other incidents involving drones and Boeing 737 planes near Stansted.

In March a pilot reported a drone 200ft away while approaching over Hertfordshire at 2,000ft.

A purple drone was also spotted by a pilot 3-4m above the aircraft at 4,000ft in the Great Dunmow area in September, and came to within 50m of colliding with the aircraft.

Neither drone operators in the two incidents could be traced.

A Stansted Airport spokesperson said: “Drones pose a serious risk when flown near airports.

“Owners of drones are legally responsible for their safe flying, and could face prosecution if they breach CAA guidelines – which includes a complete ban on their use in the vicinity of airports, unless prior permission has been given.”

A NATS spokesman echoed the comments from the airport, adding: “Flying any kind of drone near an airport or in controlled airspace without the proper permissions is dangerous and unacceptable.

“People using drones should apply common sense when deciding where to fly and need to remember that the same legal obligations apply to them as well as any other pilot.”

In a separate incident, two passenger planes using Stansted Airport came within 700ft of each other after an air traffic controller gave instructions to the wrong aircraft.

The near miss happened north east of Southend on April 1, and involved a Boeing 737 leaving Stansted and Boeing 777 heading in.

The B737 was given instructions for another aircraft by the London controller, causing it to “climb into confliction” with the B777, the UK Airprox report said.

The B737 pilot said if he had not selected altitude hold it could have been a “more serious situation”.

Risk of a collision, in his opinion, was of “medium-high risk”.

A formal assessment found the controller was “under pressure”, and “subject to a high workload” and at the time felt “overloaded”.

But although last week’s report recognised the controller’s actions caused the incident, he had “taken action” to control the situation.