The chief suspect in the new ISIS video threatening Britain and its PM David Cameron seems to be Britisher Siddhartha Dhar.

BBC reports that Dhar was the focus of investigations into the video, which purports to show the killing of five men ISIS says were spies.

"A lot of people think it is him," a source said to BBC, although there has been no official confirmation.

Dhar was a Hindu who converted to Islam and joined the Islamic State. Before this, he had joined the radical Islamist group al-Muhajiroun, which is now banned in the UK. He is now known as Abu Rumaysah. He was arrested in 2014 but jumped bail, says the report.

Al-Muhajiroun, founded by British extremist preacher Anjem Choudary, was banned a few years ago, leading to the formation of Islam4UK. Choudary is currently awaiting trial in the UK for helping garner support for ISIS.

Related Read: Britain says ISIS video threatening David Cameron shows 'barbarity of Daesh'

Hailing from Walthamstow in east London, Siddhartha Dhar is a father of four children. After jumping bail, he fled to Syria to join Islamic State.

A few years ago, when Anjem Choudary threatened a rally in New Delhi to 'force' the government to implement Sharia throughout the country, Abu Rumaysah had posted this video:

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".

Siddhartha Dhar aka Abu Rumaysah would regularly attend Al-Muhajiroun demonstrations against the US, Israel, Arab regimes or any other cause the group believed to be un-Islamic.

He would stand outside mosques on Friday afternoons, seeking to find new followers to the radical network's way of thinking. He would post videos online and rarely turned down an opportunity to speak to the media, promoting his radical views.

At least 700 people from the UK have travelled to support or fight for jihadist organisations in Syria and Iraq, say British Police.