The pilot of a Cessna plane watching a glider more than a mile away probably failed to see a second 130ft (40m) below him, investigators found.
The near miss happened over Papworth Everard in Cambridgeshire on the afternoon of 29 October.
The glider was travelling from the Nene Valley Gliding Club in Upwood towards nearby Gransden Lodge airfield and the Cessna was on its way to Cambridge.
The UK Airprox Board report concluded aircraft safety was compromised.
The near-miss was recorded on radar, but air traffic controllers were not aware of how close the aircraft were from each other until after an inquiry.
Air traffic control could identify the Cessna from its transponder, but not the glider, although radar was able to record automatically how close the aircraft passed each other.
"Based on the Cessna pilot's report and its disparity with the glider pilot's, the board suspected that the Cessna pilot had seen a different glider to the Centrair Pegase... which it passed at 130ft (40m)," the report said.
"The board then surmised that because he probably did not see the Centrair Pegase, he would not have been able to avoid it and that would explain why the glider pilot reported that he [the Cessna pilot] appeared to maintain his course and altitude as they passed each other."
The report concluded that the level of risk of collision was B, which means safety was not assured and the safety of the aircraft may have been compromised.