An official source told the BBC Mr Dhar was the focus of investigations into the video, which purports to show the killing of five men IS says were spies.
“A lot of people think it is him,” the source said, although there has been no official confirmation.
Mr Dhar – also known as Abu Rumaysah – was arrested in 2014 but jumped bail.
The former businessman and father-of-four, from Walthamstow in east London, then made his way to Syria. He had been arrested on suspicion of encouraging terrorism.
Mr Dhar was a Hindu who converted to Islam and joined the radical group al-Muhajiroun.
Earlier, one of his associates told the BBC he had “no doubt” that the voice on the video was that of Mr Dhar.
Mr Dhar’s sister told the BBC that when she first heard the audio of the video she feared it was her brother, although having watched a clip she was now not so sure.
“I was in a state of shock,” said Konika Dhar.
“I believed the audio to resemble, from what I remember, the voice of my brother but having viewed the short clip in detail, I wasn’t entirely convinced which put me at ease”.
In the latest video by the extremist group – which has not been independently verified – the masked man, who is holding a gun, mocks Prime Minister David Cameron for daring to “challenge the might” of the extremist group.
He goes on to say: “We will continue to wage jihad, break borders and one day invade your land where we will rule by the Sharia.”
Mr Cameron said the video was “desperate stuff” from a group that was “losing territory”.
The 10-minute video also features a boy with a British accent. After the apparent killings, he is seen pointing into the distance and talking about killing “unbelievers”.
Channel 4 News has reported that a south London man they named as Sunday Dare recognised the boy in the video as his grandson, Isa Dare, who was taken to Syria by his daughter Grace Dare.
“It’s my grandson, I can’t disown him, he’s my grandson. I know him very well,” he said, adding that he was being used for propaganda and “as a shield”.
He said his daughter “should come back and face the music because she has let herself down”.
Grace Dare, who also goes by the name Khadijah, comes from a British-Nigerian background and converted to Islam when she was 18. She travelled to Syria in 2013, aged 22.
In the video, five men, wearing jumpsuits and kneeling in a desert location, appear to be shot in the back of the head, after making what is claimed to be their confessions. IS says they were spying for the UK.
One of the men says he had been asked to provide information about the location of IS militants, including two Britons, apparently to help target them with air strikes.
IS has previously released propaganda videos of killings, including footage showing the apparent beheading of two US journalists, James Foley and Steven Sotloff, and two British aid workers, David Haines and Alan Henning.
Mohammed Emwazi, the Briton who became known as Jihadi John, appeared in the videos. The US said he was killed in a drone strike it carried out in Syria in November.
The IS group, notorious for its brutality, seized large swathes of territory in Syria and Iraq in 2014, when it formally declared the establishment of a “caliphate” – a state governed in accordance with Islamic law, or Sharia, by God’s deputy on Earth, or caliph.
At least 700 people from the UK have travelled to support or fight for jihadist organisations in Syria and Iraq, British police say. About half have since returned to Britain.