As the two Russian SU-27s approached the RAF spy plane, they received a communication from their ground station controller.
One Western source told the BBC the words they received were to the effect of "you have the target".
This ambiguous language was interpreted by one of the Russian pilots as permission to fire.
The loose language appears to have shown a high degree of unprofessionalism by those involved, sources said. In contrast, Nato pilots use very precise language when asking for and receiving permission to fire.
The Russian pilot released an air-to-air missile, which successfully launched but failed to lock on to its target, the BBC has been told. It was a miss, not a malfunction.
Defence sources have told the BBC that a row then broke out between the two Russian pilots.
The pilot of the second SU-27 did not think they had been given permission to fire.
He is said to have sworn at his comrade, effectively asking him what he thought he was doing.
Yet the first pilot still released another missile.
We had been told that the second missile simply fell from the wing - suggesting the weapon either malfunctioned or that the launch was aborted.