Two military transport planes came within 1,700ft (520m) of colliding near an RAF base, a report has said.
The near miss involving two Airbus A400s in March happened when an air traffic controller instructed the wrong aircraft to descend.
The controller at RAF Brize Norton had not received a handover from the radar controller, due to "a very high workload", the report found.
It said this was "a contributing factor" to the near miss.
One of the planes was leaving RAF Brize Norton in Oxfordshire, as the other was descending to land at the base.
The pilot of one of the planes said without taking avoiding action there was a "definite possibility of a mid-air collision".
The UK Airprox board, which investigates near collisions between aircraft, has recommended RAF Brize Norton's air traffic control reviews how tasks are assigned.
It classified it as a category C incident, which means "no risk of collision has existed or risk was averted".
The report said some members of the board felt it should have been assessed as category B because safety "was much below the norm".
An RAF spokesman said it welcomed all the recommendations made in air proximity reports.
He added that such cases were rare in comparison to the "millions of military and civilian flights" made in UK airspace each year.