Voice recognition experts working for the security services were understood to be urgently scrutinising the Isis video amid the suggestion that the Islamist's voice matched that of Dhar. Analysts will be able to glean a huge amount of information, including a possible positive identity from the gunman's distinctive accent, language and tone. Dr Frederika Holmes, a forensic speech and voice analyst said the sound on the video was of such good quality that experts would be able to compare very accurately against existing audio clips.
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Dhar's sister, Konika Dhar, said she did not want to believe that her brother could be responsible for the outrage, but conceded that the voice on the video "did sound like him".
She 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 he had converted more than 10 years ago, and her memories of her brother are from when they were children and teenagers. "He was a very pleasant boy, and I know it may be hard to believe but he still is, and I still believe that he still can be that person."
Dhar, who was brought up a Hindu, ran a firm hiring out bouncy castles, before marrying his Muslim wife Aisha and becoming radicalised.
Meanwhile, the grandfather of the 4-year-old London boy in the video has spoken of his disgust after the child was used as a propaganda tool.
Isa Dare, whose mother Grace wants to become the first British woman to execute a Western hostage, talked in the film of killing "kuffars" or non-believers.
Grace Dare, 24, who changed her name to Khadijah after converting to Islam as a teenager, is a former media studies student who is suspected of producing the slick video, in which five hostages are shot dead before Isa appears in the final seconds, dressed in camouflage gear and a black Isis headband.
Grace Dare's father Henry, who confirmed that the boy in the film is "definitely" his grandson, said: "Of course I'm worried but there is nothing I can do now. I just hope someone is trying to bring them back."
In a direct appeal to his daughter, who travelled to Syria in 2012 to become a jihadi bride, he said: "Grace, Grace, come home with my grandchildren. Come home, we love you. Come and face the music."
He also told Channel 4 News: "He doesn't know anything, he is just a small boy. They are just using him as a shield. It's propaganda. "
Grace Dare, who was born in Lambeth to Nigerian immigrant parents in 1991, is a widowed mother-of-two whose second husband, a Swedish jihadi called Abu Bakr, was killed.
Writing under the Twitter name Muhajirah fi Sham (immigrant in Syria) she said after the murder of US journalist James Foley in 2014: "Any links 4 da execution of da journalist plz. Allahu Akbar. UK must b shaking up ha ha. I wna b da 1st UK woman 2 kill a UK or US terrorist!"
She also posted a picture of Isa, aged around 3, aiming a Kalashnikov automatic rifle he was barely big enough to lift.Telegraph Group Ltd
