David Cameron under pressure to explain how Isis video suspect was able to skip bail and flee to Syria

Siddhartha Dhar mocked the 'shoddy security system' which allowed him to travel to Syria. His sister says his voice is 'a bit like' that of the militant in Isis's new video - but she has her doubts that it is really him

British security forces are facing mounting pressure to explain how the lead suspect in the hunt for the masked militant featured in Isis’s latest execution video was able to skip bail and leave the UK.

The matter will be raised in the Commons after the shadow Home Secretary, Andy Burnham, tabled an urgent question on what he called the "serious lapse in security".

Siddhartha Dhar was being investigated by the Metropolitan Police for terror charges when he escaped the UK in 2014, and tweeted afterwards that Britain’s “shoddy security system… allow[ed] me to breeze through Europe to the Islamic State”.

Scotland Yard has told The Independent it is closely following developments on the footage which, posted online by Isis’s Raqqa media wing, shows a British-accented man ranting against David Cameron before he and others appear to shoot dead five hostages.

And Boris Johnson, the London Mayor, has said he will speak to police to find out how the decision was made to grant Mr Dhar bail in the first place.

The masked militant taunts David Cameron in Isis's video, before shooting a hostage

Last night, Syria-based activist group Raqqa Is Being Slaughtered Silently added its voice to those saying they believe Siddhartha Dhar, now going by the name Abu Rumaysah, is the masked man at the centre of the new video.

British officials have told the BBC that after two days of analysis by MI5, MI6 and GCHQ, “a lot of people think it is him” though there has been no confirmation.

Mr Dhar’s own sister, Konika Dhar, has told media outlets that the voice of the fighter in the video is “a bit like his”, but said some doubts remain in her mind.

She told Sky News: "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. My inclination is that it most likely may not be."

A former bouncy castle salesman from east London, the 32-year-old British-Indian Mr Dhar was closely linked to the proscribed extremist group al-Muhajiroun.

He repeatedly appeared across broadcast media in the months before he fled the country, making no attempt to hide his radical views and only stopping short of saying he was willing to fight for his beliefs “because I would be arrested”.

Shortly after arriving in Syria with his pregnant wife, Mr Dhar posted images to social media of himself cradling his newborn baby in one hand, and an assault rifle in the other.

In spring 2015 he penned a 46-page “holiday brochure” comparing Isis’s territories to a “plush holiday resort”, which experts dismissed as so clearly false as to be “juvenile” and “weak in terms of its propaganda value”.

In a Twitter account which has since been suspended, Mr Dhar praised Allah “that He facilitated a path for me and my family to live in the Islamic State. Inshaa’Allah (God willing) we will defeat the final Crusade”.