The Foreign Office has said it is examining a video purporting to be by so-called Islamic State which shows a man and young boy with British-sounding accents and the killing of five men IS says were spying for the UK.
The disturbing Islamic State propaganda video shows a masked Jihadi with an apparently British accent threatening David Cameron and the UK with new terror attacks.
In the 10-minute long video, five shackled men wearing orange jumpsuits are shot dead after the masked man condemns British efforts to destroy IS.
Before their murders, the men are shown 'confessing' in Arabic to spying for Britain.
Addressing the camera during the clip, the man calls Prime Minister David Cameron "a slave of the White House", saying:
One would have thought you would have learnt the lessons from your pathetic master in Washington and his failed campaign against Islamic State.
The English-speaking murderer dismisses the UK as a "small island" and mocks the British contribution to coalition airstrikes against the death cult.
"How strange it is that we find ourselves today hearing an insignificant leader like you challenge the might of the Islamic State," he said.
How strange it is that the leader of a small island threatens us with a handful of planes.
The chilling footage, which ITV News has decided not to show in its entirety, also shows a young boy, dressed in a combat uniform.
The youngster points to the distance and declares in a seemingly British accent: "We will kill kaffir [non believers] over there".
The video has a grim resemblance to earlier propaganda featuring Mohammed Emwazi, the British IS militant otherwise known as "Jihadi John", who brutally murdered several hostages including Britons David Haines and Alan Henning.
A Foreign Office spokesman said: "We are aware of the video and are examining its content."
ITV News has not so far been able to independently verify the video but it comes from a source that has proved to be accurate in the past.