Five men, who the group claim were British operatives, are executed in the video
Video has emerged purporting to show a masked Islamic State militant with a British accent threatening the UK and calling David Cameron an "imbecile".
In the footage, five men of Middle Eastern appearance and wearing orange jumpsuits, who IS said were British spies, were shot dead.
A child, who also apparently spoke with a UK accent, later appeared in the video wearing an IS headband and in a camouflage outfit.
The masked adult, who is also in combat fatigues, was seen behind the five men and called the UK Prime Minister a "slave of the White House".
The fighter said: "One would have thought you would have learnt the lessons from your pathetic master in Washington and his failed campaign against Islamic State.
"But it seems that you, like your predecessors Blair and Brown, are just as arrogant and foolish.
"In fact David you are more of an imbecile."
He added: "You will lose this war as you lost in Iraq and Afghanistan."
The RAF has carried out at least 10 airstrikes against Islamic State in Syria since MPs backed the use of military force early in December.
UK warplanes have also been bombing IS targets in Iraq for a number of months.