An RAF training plane had to take evasive action when it ventured in to airspace used by a Cambridgeshire model flying club last year.
A UK Airprox Board report labelled the incident "high severity" after the trainer flew within 40ft (12m) of a remote-controlled model aircraft.
The near-miss happened 600ft (180m) above the village of Northborough near Peterborough.
The board said the "safety of the aircraft may have been compromised".
A report just published said the planes were flying on 4 February 2014 in airspace permitted for use by a local model aircraft flying club.
The model's operator thought his plane was flying at up to 150ft (45m), but the trainer, from RAF Wyton near Huntingdon, confirmed it had been flying above it at one point at 600ft.
The Airprox Board report concluded "safety margins had been much reduced".
The board suggested because nearby RAF Wittering had changed to a non-flying base users of the airspace had become less aware of dangers.
It said all operators in the area needed to be alert to each other's activities.
The Ministry of Defence said RAF Wittering had since returned to flying status.