A new Islamic State video purports to show the death of five alleged British spies at the hands of a man speaking English with a British accent. And, though the video was not yet authenticated, news outlets were already comparing the grisly spectacle to the work of Jihadi John.

A 10-minute video shows a masked militant with a British accent threatening the UK, and footage of five men of Middle Eastern appearance wearing orange jumpsuits – who IS said were British spies being shot dead.

Security officials are examining the footage, first released through the SITE Intelligence Group, to try to identify the masked man. He appears to mimic the style of the British man known as Jihadi John (real name – Mohammed Emwazi) who was killed in a US drone strike in Syria in November. Gesticulating in a manner similar to that used by Emwazi, he repeatedly waves a gun at the camera as he references British airstrikes in Syria and labelling David Cameron “an imbecile”.

The chief suspect has been named in some media reports as Siddhartha Dhar – a Muslim convert from Walthamstow, east London.

Siddhartha Dhar, who is also known as Abu Rumasayah, reportedly ran a business renting out bouncy castles in London before converting. He was arrested in September 2014 with eight other men as part of an investigation into the banned extremist group al-Muhajiroun. He fled Britain with his family the following day after being released on bail and travelled to Syria, where he joined ISIL.

The mother and sister of Siddhartha Dhar said they had watched the latest Islamic State of Iraq and the Levant (Isil) video, released by ISIL on Sunday. The sister of Mr Dhar has told the BBC that when she first heard the audio of the video she feared it was her brother, although having later viewed the pictures, she was not entirely convinced it was him.

His sister, Konika Dhar, from north London, said she did not want to believe her brother could be responsible for the outrage, but conceded the voice on the video “did sound like him” and that she is not sure if it is him. She also said he had converted more than 10 years ago, and her memories of her brother are from when they were children and teenagers. “My honest opinion is that it might not be him. The features are quite different and from what I remember of my brother he doesn’t have the strong eyebrows.”

A senior member of the banned Islamist extremist group al-Muhajiroun, however, has told the BBC he “has no doubt” that the voice is that of Mr Dhar. Voice recognition experts working for the British security services were scrutinising the ISIL video amid the suggestion that the Islamist’s voice matched that of Dhar.

Siddhartha’s mother, Sobita Dhar, said she could not rule out that the man in the video was her son. She said: “I heard the voice, yes, but I don’t know, I’m not sure of the voice. These are the most difficult questions to answer. I just cannot say,” and “I’m not sure within myself whether it is the truth or not.”

Konika Dhar said: “I can’t believe it. This is just so shocking for me. I don’t know what the authorities are doing to confirm the identity, but I need to know if it is.” She said: “If it is him, bloody hell, am I shocked? I am going to kill him myself. He is going to come back and I am going to kill him if he has done this.

Konika also pleaded with the UK authorities to provide an alternative for those who have been radicalised and travelled abroad, to help them return home.

Meanwhile, a child who also featured in the video was positively identified as the son of jihadi bride Grace Dare, from Lewisham, south-east London.

Watch the video here:

The Isil propaganda video released on social media on Sunday shows five men, wearing jumpsuits and kneeling in a desert location, who appear to be shot in the back of the head, each confesses to passing on information about Isil and its operatives. In the video, the masked gunman warned Mr Cameron that the West could never win the war against Isil.

The terrorist also claims that one day Britain would be invaded, with everyone forced to live under strict Sharia law. British Prime Minister David Cameron denounced the video as “despicable” but said people should watch it remind themselves of Isil’s “ghastly” tactics.