An Islamic State video featuring a masked British extremist and a little boy is being examined by UK security services.
The footage released online shows the brutal murders of five men accused of spying for the UK.
A jihadi is seen waving a gun as he mocks Prime Minister David Cameron and the RAF bombing campaign against IS in Iraq and Syria.
Five men in orange jumpsuits "confess" to filming and photographing sites in exchange for money within Raqqa, the capital of IS's self-declared caliphate.
The militant claimed the Government had "abandoned these spies" as the men are seen kneeling before they are shot in the head.
Following the killings the 10-minute clip ends with a young boy dressed in military fatigues and wearing a black headband.
Speaking with what sounds like a British accent the child points into the distance and says in English: "We are going to go kill the kaffir (non-believers) over there."
IS - also known as Daesh, Isil and Isis - has used children in a number of previous propaganda videos.
The latest film bears similarities to those which featured the British jihadist known as Jihadi John.
The killer, real name Mohammed Emwazi, appeared in videos showing the murders of Britons David Haines and Alan Henning, plus those of US journalists James Foley and Steven Sotloff.
Emwazi was killed in a US drone strike in Syria in November.
It comes days after Mr Cameron promised to crack down on IS sympathisers, stressing in a New Year's message that all Britons should have "loyalty" to their country.
Pointing a handgun, the masked militant addresses the Prime Minister, saying: "How strange it is that a leader of a small island threatens us with a handful of planes. One would have thought you would have learned the lessons of your pathetic master in Washington and his failed campaign against the Islamic State.
"But it seems like you, just like your predecessors (Tony) Blair and (Gordon) Brown are just as arrogant and foolish. In fact David, you (waves handgun) are more of an imbecile. Only an imbecile would dare to wage war against a land where the law of Allah reigns supreme and where the people live under the justice and security of the sharia.
"Only an imbecile would dare to anger a people who love death the way you love your life."
The jihadi goes on to say that IS is "here to stay" and will invade the UK and rule under sharia law.
Whitehall sources suggested the video was an attempt to distract attention from recent military setbacks IS has faced, like losing the Iraqi town of Ramadi to state forces.
A Foreign Office spokesman said: "We are aware of the video and are examining its content."
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