A masked Briton has appeared in what is believed to be a new Islamic State (IS) video showing the brutal murder of five men accused of spying for the UK.
The footage released online shows the jihadi - with a British accent - waving a gun as he mocks Prime Minister David Cameron and the RAF's bombing campaign against IS in Iraq and Syria.
The footage, which has yet to be independently verified, shows five men in orange jumpsuits "confessing" to filming and photographing sites in exchange for money within Raqqa, the capital of IS' 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.
The video bears similarities to those which featured the British jihadist known as Jihadi John. The killer, real name Mohammed Emwazi, appeared in videos showing the execution 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.
The 10-minute recording also features a small boy.
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 jihadi addresses Mr Cameron, 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 Blair and 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."
Whitehall sources suggested the latest video was an attempt to distract attention from recent military setbacks IS has faced, like losing the Iraqi town of Ramadi to state forces.
This was echoed by security expert Shashank Joshi, who said it was aimed at "threatening, goading and provoking" IS's opponents in the West.
Mr Joshi, a senior research fellow at the Royal United Services Institute (RUSI), said the sight of a masked executioner declaring in a British accent that David Cameron is an "insignificant leader" who dares to "challenge the might of the Islamic State" was timed to strike terror internationally while the extremists have suffered several tactical setbacks on home ground.
"I think there is an overall sense in which they have had relatively few successes in the past few weeks," he said.
"They have been pushed back and these videos are a method to try and maintain support."
The jihadi goes on to say that IS will invade the UK and rule under sharia law.
He adds: "As for those of you who wish to continue fighting under the banner of Cameron, on the minimum wage, we say to you, to ask yourself, do you really think your government will care about you when you come into our hands? Or will they abandon you, as they have abandoned these spies and those who came before them? Because you will lose this war, as you lost in Iraq and Afghanistan."
London mayor Boris Johnson called the video "appalling".
Writing in the Telegraph, he said: "Our age is likely to be bedevilled by anxiety about Islam - or at least about Islamism. Consider the psychological impact of last night's appalling video message from the sick fanatics in Raqqa.
"We will be forced constantly and ruthlessly to insist on the distinction between Islamic extremism and a religion that is followed by more than a billion people who are no less peaceful, no less loving, no less kind or good than ourselves."
And Colonel Richard Kemp, a former commander of British forces in Afghanistan, accused the masked executioner of "cowardice".
He told the Daily Mirror: "No doubt there will be subsequent appeal to British Muslims to attack other British people. The inclusion of a child is designed to shock us as it has done in previous videos of children holding heads up and things like that.
"This gunman is trying to look tough. But he conceals his face and is obviously scared of being killed by our forces. This is exactly the kind of cowardice we have come to expect from people like him."
The Prime Minister said in his New Year message that 2016 would be a "test of our mettle" as he pledged action to tackle the "poisonous narrative" which led some Britons to turn against their country.
He said the UK should "revel" in its way of life rather than "appease" extremists, and all who live in the country must sign up to its values.
Mr Joshi said the presence of a child would be a another cynical move by IS because "they use their messengers very carefully".
He said: "It is more manipulation of us and therefore more threatening.
"The child would be a reminder of the danger of IS and also of how they see themselves as a nation building process. They have indoctrinated families and children under their control and also indoctrinated children to do brutal things.
"It is a reminder of how they use children as a threat and to further their propaganda."
According to the Guardian, the five victims were Ha'il Marwan Abdul Razaq, 40, Ubi Muhammad Abdul Ghani, 26, Faisal Hamud al-Ja'far, 25, Umaar Hamud al-Ja'far, 30, all from Syria, and Mahyar Mahmud al-Uthmaan, 31, whose origin is unclear.
A Foreign Office spokesman said: "We are aware of the video and are examining its content."