Intelligence agencies were hunting a new "Jihadi John" after an Islamic extremist with a British accent was shown directing the murders of five men accused of spying for the United Kingdom.
In a video released on social media yesterday, the masked gunman warned British Prime Minister David Cameron that the West could never win in the war against Isis (Islamic State), while mocking the impact of RAF airstrikes.
At the end of the video a young boy aged about 4, who seemed to speak with a British accent, also appeared on screen, threatening to kill non-Muslims.
The victims - who are said to have helped coalition forces by providing information about the whereabouts of Isis suspects, including the group's deputy leader - were all shown being shot in the head at point-blank range after the British-sounding terrorist delivered his message of propaganda.
The 10-minute video had echoes of those posted by the Isis terrorist dubbed "Jihadi John", who was responsible for a string of murders including that of British aid worker Alan Henning. "Jihadi John", later identified as 27-year-old Mohamed Emwazi from London, was killed in a United States drone strike last November.
MI5 agents were working to establish the identity of the new terrorist, amid concern that the video might signal the start of a new Isis propaganda campaign, in the wake of Britain's decision to join coalition airstrikes against Isis targets in Syria.
A source in the Foreign Office said the propaganda video had likely been released to "divert attention away from Isis' recent military failures in Iraq and its inability to look after the citizens in the areas it controls".
The terrorist made reference to the soldiers being paid the minimum wage, and labelled previous Prime Ministers Tony Blair and Gordon Brown "arrogant and foolish" for the use of British troops in Iraq and Afghanistan.
The video began with apparent confessions from the five Arabic men who were wearing orange prisoner-style jumpsuits. The men claimed to have passed information to the West about prominent Isis figures who were subsequently targeted by the coalition, including the group's deputy leader Abu Muslim al-Turkmani, who was killed in a US drone strike last August.
The video then cut to a desert area where the victims were shown kneeling before five gunmen clad in camouflage clothing and black masks.
Addressing his rant to Cameron directly, the terrorist described him as a "slave of the White House" and "mule of the Jews".
Clutching a semi-automatic pistol he said Cameron was an "imbecile" for thinking RAF airstrikes could have any impact. "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 Sharia." Then, addressing a wider audience, the terrorist warned that one day Britain would be invaded with everyone forced to live under strict Sharia law.
After then shouting Allahu Akbar, Arabic for "God is great", he then opened fire on the victim at his feet, shooting him in the back of the head.
The man slumped forward before the other gunmen all opened fire on their victims in a sickening killing spree. After a montage featuring Isis propaganda slogans and music, the video cut to another desert landscape, where the boy addresses the camera.
Wearing a camouflage top and a black Isis headband, the youngster appeared to speak with a British accent as he pointed to the distance and said: "We will kill the kuffar [non-believers] over there," as he points to the distance.
While his accent was difficult to ascertain, the decision to use the child in the video suggested a determination by the Isis propaganda machine to further shock and terrorise the West.
Envoy of evil - executions an attack on activists inside 'caliphate'
The so-called "caliphate" has been targeting a group called Raqqa is Being Slaughtered Silently (RBSS), an alliance of journalists and activists from the Syrian town that is the jihadists' de facto capital.
The five men killed in the video "admit" providing information to men who take it to Turkey where it is released to the Western media.
It names a man called Manal Abdul Razzak as being both an Isis fighter and an "an agent of the Crusader alliance". His brother, Marwan Abdul Razzak, is one of the men who confesses to providing information to RBSS and is then shown being shot.
RBSS is not named but some of its members are, including Hamoud al-Mousa, one of its founders.
"I started to work with Hamoud al-Mousa and he asked me to work inside Raqqa," says another of the men, Oubai Mohammed Abdul Ghani, 17.
"He asked me to take various videos inside Raqqa and videos about women in Raqqa to produce a documentary against the Islamic State."
He says that he was then asked to provide information on a series of Isis buildings and on individuals, including one Australian jihadi and two Britons.
They are not named, but he refers to a subsequent airstrike which struck a car in the city, near the state security building. Mohammed Emwazi, the British Isis spokesman and executioner known as "Jihadi John", was killed in a strike on the city as he was getting into a car.
Abdul Ghani makes the most important single confession of the video, saying that he was sent by Hamoud al-Mousa a photograph of "Abu Muslim al-Turkmani" and asked to monitor him.
Abu Muslim al-Turkmani - real name, Fadel Ahmed Abdullah al-Hiyali, and at one time a colonel in the army of former Iraqi leader Saddam Hussein - was deputy leader of Isis and in charge of its Iraqi operations until he was killed in an American drone strike in August.
The success of the coalition in hitting and killing two of its "most wanted" in quick succession - Emwazi was killed in November - suggested that Western intelligence on Isis had improved and had even managed to penetrate the group sufficiently to monitor individuals.
The video would indicate that this success has sent some panic through the group. It has suffered on three fronts in the last six months. On the battle-front, it has been forced out of the key cities of Ramadi and Sinjar in Iraq and has been on the retreat in northern Syria, where a route from Raqqa to the border is under threat.
Internally, a number of its leaders and middle management have been killed in targeted strikes.
Thirdly, it has suffered on the propaganda front, with some indications that the recruitment appeal of spectacular attacks such as in Paris in November is being outweighed by the increased determination among Isis' opponents to unite to fight it.
RBSS has also damaged Isis' reputation among Muslims who might have been tempted to join it, portraying life inside the "caliphate" as less glamorous than those who initially glorified it as "five-star jihad" would have them believe.
Its videos and leaks showed a world of severe social restrictions, diminishing resources, and both military and civilian casualties from bombing raids.
For Isis, trying to demonstrate RBSS as a front for Western intelligence is a natural response. The best explanation for the video may perhaps be Isis' annoyance at al-Mousa.
A brother and fellow RBSS activist, Ahmed al-Mousa, who was said to have been the link for two brothers killed in the video, was shot dead by an unknown group of masked men in Idlib province, in rebel-held north-west Syria, two weeks ago. He is one of at least five members of the group to have been murdered.
This video may ostensibly be aimed at Britain, and use Isis' customary "shock tactics" to prove to the West that it is still able to provoke horror and fear.
But it also has the practical, closer-to-home purpose of warning those living under its sway of the perils of dissent and disloyalty.