Security services and police are attempting to discover the identity of the people behind a new Islamic State (IS) video believed to have British links.
It came as the sister of an alleged jihadi said she will "kill him myself" if Abu Rumaysah is revealed as the masked militant in the footage showing the murder of five hostages.
Konika Dhar, from north London, said she does not believe her brother is the man in the film, but she conceded the voice sounded “a bit like him”.
The law student pleaded with British authorities to help her brother and others who have been radicalised so they can return to the UK, saying they are “trapped” and have no alternative but to “kill or be killed”.
Ms Dhar said she has not had contact with Rumaysah for more than a year, admitting she "doesn't even know who he is any more". Describing the brother she remembered as "very pleasant, very amicable, very friendly", she said she was desperate to help "de-radicalise" him. Ms
Dhar said she had seen around three minutes of the video posted online over the weekend by Islamic State (also known as Isis) in which a man thought to be her brother goads Prime Minister David Cameron.
She said she was not convinced the man was her brother, saying: “I don’t think it is, personally, to be honest.” She went on: “I examined the features — one eye is smaller than the other, my brother has symmetrical eyes. The eyebrows are bushy and this guy is taller, my brother is shorter and he has got broader shoulders, but he has got stooped shoulders so I don’t think it is.
“I think the voice sounds a bit like him, but it could be someone else, but he could have helped with a voiceover, but I have no idea.”
It came as a small boy who appeared in the footage was named in reports as the son of a jihadi bride from south-east London.
The identities of the people in the 10-minute clip, which is being closely examined by security services, have not been confirmed.
Rumaysah previously talked about the UK being brought under sharia law by a conquering Islamic State army.
Born a Hindu called Siddhartha Dhar, he ran a business renting out bouncy castles in London before converting.
He was arrested in September 2014 when he was 31, with eight other men, as part of an investigation into alleged support of the banned extremist group al-Muhajiroun.
But in a huge embarrassment for authorities he fled Britain with his family the following day after being released on bail, reportedly travelling to Paris and then Syria.
Ms Dhar said she has not had contact with her brother in more than a year, saying she “doesn’t even know who he is” any more.
Ms Dhar 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.
“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.”
Meanwhile, reports claimed a young child seen at the end of the video dressed in military-style clothes and wearing a black headscarf with an Islamic State logo on it was Isa Dare, the son of Grace "Khadija" Dare, from Lewisham, south-east London.
The footages shows the boy pointing into the distance and saying in English: “We are going to go kill the kaffir (non-believers) over there.”
Grace Dare was brought up as a Christian but converted to Islam and started using the first name Khadijah. She travelled to Syria in 2012 and married a Swedish man called Abu Bakr, who is since believed to have been killed.
Last night, Isa's grandfather Henry Dare told Channel 4 News the boy was used for propaganda.
He said: “He (Isa) doesn’t know anything. They (IS) are just using him as a shield. She (Khadija) should come back and face the music because she has left herself down. All of us, she’s let us down.”
David Cameron described the video as “desperate stuff” from a group that is “losing territory”.
He said he had viewed part of the video, adding: “I think it is absolutely desperate tactics from an organisation that is clearly now on the back foot.”
The footage shows five men in orange jumpsuits “confessing” to taking money in return for filming and photographing sites in Raqqa, the capital of IS’s self-declared caliphate.
The militant in the film claims the UK Government “abandoned these spies” as the men are seen kneeling before being shot in the head.
The five victims have been named in reports as 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.