The SAS marksman fired one bullet that killed two men instantly before it ricocheted into a third during a November mission in a remote northern Iraqi village.
The sniper fired his single .338 Lapua Magnum bullet from a L115A sniper rifle from a range of 1,800m just as the senior ISIS members prepared to fire shots into a crowd of women and children.
SAS intelligence gathering as the troops monitored Islamic State-controlled areas while Iraqi forces advanced toward Mosul.
The SAS squad was tracking the IS members when dozens of women and children tried to flee from the extremists.
When the terrorists ordered the civilians to stop and pointed a machine gun at the crowd from a second-storey window, the SAS team decided they had no choice but to open fire and save the fleeing group.
The sniper, whose rifle was fitted with a suppressor to reduce noise, fired his once-in-a-lifetime shot.