Pilot 'did not have time to react' in near-miss over Wittering
A glider passed close to another aircraft before its pilot "had time to react" in a high-risk near-miss, an inquiry found.
The incident, involving a Shark pilot and glider, near RAF Wittering, in Cambridgeshire, was graded Category A by the Airprox Board - the most serious rating.
Investigators said the glider pilot could not be traced after the incident.
They said it was "highly probable" the glider had not seen the plane.
The Airprox Board report said the Shark pilot was aware there was other traffic in the Wittering airspace on 13 May and was "actively looking for it".
It said the glider passed about 30m (98ft) above and between 5m (16ft) and 10m (33ft) to his left.
The pilot assessed the risk of collision as "high".
The Airprox Board, which investigates near misses, said it was "unfortunate" the glider involved was not known to the Wittering controller and could not be called.
It said the glider passed just above the Shark aircraft, before the pilot had time to react, effectively making it a "non-sighting".
The board said glider pilots were encouraged to call air traffic control units when passing nearby and it was "unfortunate" this pilot had chosen not to.
However, the board acknowledged the glider may not have been fitted with a radio, and that many glider pilots did not hold a radio licence.
It said there had been a "serious risk of collision".