ISIS militants have released a video purporting to show the killing of five men they claim were spying for Britain, accompanied by a message taunting Prime Minister David Cameron for challenging the "might" of the extremist group, also known as Islamic State or IS.
The Foreign Office said it was studying the contents of the 10-minute film, in which a man speaking with a British accent threatens attacks in the UK. A boy, also apparently speaking English, appears in the video — which has not been independently verified — talking about killing "unbelievers".
The propaganda film appeared less than two months since Mohammed Emwazi, a British ISIS militant who appeared in videos depicting executions, was killed in a US drone strike in Syria.
The latest video appears to show another man trying to assume the former role of Emwazi, also known as Jihadi John. It will be studied intensively by British intelligence.
ISIS believes that spies working for coalition forces helped to locate Emwazi and are operating on the ground in the areas it controls.
In the new video a masked man, holding a gun, taunts Mr Cameron for daring to "challenge the might" of ISIS and threatens British people, saying that the militant group will "one day invade your land".
Speaking to the camera, the masked gunman can be heard saying: "This is a message to David Cameron," whom he describes as an "imbecile" and "slave of the White House".
Five men, wearing orange jumpsuits, are then seen being shot in the back of the head in a desert location, after making what is claimed to be their confessions.
One of the men says he had been asked to provide information about the location of ISIS militants, including two Britons, apparently to help find targets for air strikes.
Some of the five men say they are from Raqqa in Syria while another says he is from Benghazi, Libya, but none say they are from the UK.
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In the accompanying message, the masked militant compares the prime minister to "arrogant and foolish" predecessors Gordon Brown and Tony Blair. He concludes: "You will lose this war, as you lost in Iraq and Afghanistan."
Last month MPs voted to extend British military action against ISIS from Iraq to Syria, prompting warnings from Labour leader Jeremy Corbyn that the move would lead to Isis reprisals.
But Mr Cameron argued that Britain was already a target for the extremist group, not least because the RAF was already flying missions against Isis in Iraq, and that threat levels could not be any higher.
A Foreign Office source said the identities of the alleged "British spies" shown in the video were unknown and that the film might be purely a propaganda exercise.