The crack soldier fired a bullet designed to inflict massive damage from three quarters of a mile away and scored a direct hit on the extremist.
His rifle was so high-powered that the bullet decapitated the bearded warrior, who stood well over six feet tall.
The fearsome recruiter for ISIS, also known as Daesh, was gunned down in the middle of a drill teaching new soldiers how to execute prisoners.
'One minute he was standing there and the next his head had exploded', SAS source.
Military sources said he was demonstrating a cutting motion with his arm when the bullet hit.
Some 20 junior fighters were taking part in the outdoor lesson when their commander was killed before their eyes.
One military witness said the entire group fled in terror and deserted the terrorist cause, a source told media.
The sniper, operating in northern Syria, used a Dan .338 rifle with ammo that "tumbles" which it hits a target - inflicting a huge wound.
He had to aim more than a foot wide of the mark to adjust for wind conditions, but hit his target first time.
Suppressing equipment meant that flashes and noise from the shot were kept to a minimum and the soldier went undetected.
Although the British Army has steered clear of an all-out ground war with ISIS, highly-trained SAS squads have been deployed in small numbers.
Recently it emerged that troops were setting desert death traps for terrorists by laying out dummies dressed as enemy officers.
The victorious British sniper was supported in his mission by more than a dozen SAS comrades, Express reported.
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