TWO F-15 fighter jets came within half a second of each other in a mid-air near-miss.
The planes were flying at a combined speed of nearly 900mph at the time.
A report revealed that an air traffic controller described the risk of a collision as “high”.
The near-miss happened at 11,000ft above Cawston, near Norwich.
Both US Air Force jets were returning to their base at RAF Lakenheath, Suffolk, last October.
Radar records showed they were 800ft apart vertically, and less than 600ft horizontally.
At their speeds, it would have taken half a second to reach each other.
The UK Airprox Board, which investigates near-misses, said the pilots — who were flying as a pair — may have been distracted by a “high workload”.
It said one made an “aggressive manoeuvre” to avoid another jet — which had been flagged by air controllers.
The pilots described collision risk as “low”.
The USAF 48th Fighter Wing said it was making “a wider effort” to ensure that F-15 crews respond to controllers’ instructions.
Last April two F-15s nearly collided with parachutists in Cambs.
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