Concerns have been raised over the flight paths of Army Apache helicopters in a “hotspot” area of sky after two came within close proximity to light aircraft within weeks.
Reports by the UK Airprox Board, which investigates near-misses, outlined two incidents over Birch in Essex.
Investigators said there had been other cases and the Apache Helicopter Force should “take note of this”.
The Ministry of Defence said it took all air incidents “very seriously”.
It said it welcomed all recommendations made in Airprox reports and would “do whatever we can to prevent them from happening again”.
The reported incidents involved Apaches from Wattisham Airfield in Suffolk and a light aircraft.
On 7 August 2018, an Apache pilot reported he came within 100ft (30m) of a light aircraft, although radar suggested it was within 400ft (121m), the first report said.
The second report said an Apache pilot reported he was flying at 1,250ft (381m) to Wattisham Airfield on 26 September 2018 when he spotted an aircraft at a distance of 1,640ft (500m).
They passed within 200ft (60m), the report said.
There was “no risk of collision” in either incident.
Investigators said there had been other incidents between Apaches, which can cost at least £20m per helicopter, and general aviation aircraft using Birch to practice forced landings.
They said it ought to be noted by the military for “planning and briefing purposes”.
The report also noted some board members thought it “would have been better” if Apaches travelled to the area at a different height as 1,000ft (304m) to 2,000ft (609m) was used by light aircraft.
It said the increased use for commercial flights of Southend Airport, which has become a Ryanair base, may have pushed more light aircraft into using the route over the Birch area, described as a “potential hotspot”.
The Apache, based Wattisham, has been used for sorties in Afghanistan and Libya, where it was used to hunt and destroy tanks.