A second British extremist is feared to be among five executioners who featured in the latest Islamic State video.

The group's leader has already been identified as Siddhartha Dhar, the Londoner described as the 'new Jihadi John'.

Security sources have now told The Mail on Sunday they suspect that the tall man on his left in the film is Mohammed Reza Haque, 35, nicknamed Giant.

Before he slipped out of Britain for Syria two years ago, Haque worked as a bodyguard for a notorious preacher.

He was pictured at a rally brandishing the black IS flag alongside others who held banners declaring: 'British soldiers burn in Hell.'

In the IS video released earlier this month, the masked terrorists stand behind a line of five 'British spies' - all kneeling and wearing orange jumpsuits. They kill each of them in turn, shooting them in the back of the head.

Haque was already in Syria when Dhar, a close associate, skipped bail and joined IS in September 2014.

Officials believe the execution video might have been a 'loyalty test' for Dhar and his group. A source said that with pressure on IS mounting, paranoia had grown among members and some had begun 'turning on each other'.

Haque travelled from Stansted to Istanbul via Cyprus with another man on January 13, 2014. From there, they are thought to have crossed into Syria.

In Britain, Haque was a constant presence during radical marches, but often wore a mask to hide his identity.

The preacher he worked with cannot be identified for legal reasons. Despite Haque's prominence, he - like Dhar - was able to evade border checks, raising questions about how closely known radicals are being monitored.

A photograph of Haque in Syria shows him in a balaclava and camouflage clothing, and brandishing an assault rifle alongside an unmasked man with a similar weapon.

Dhar, 31, a Hindu convert who calls himself Abu Rumaysah, had been released on bail after being arrested for terrorism offences in September 2014.

He subsequently fled the country with his pregnant wife and four children by coach from London Victoria to France, using his own passport.

He then posed in military-style coat and boots, brandishing an assault rifle and holding his newborn baby in Syria in November 2014, labelling the picture 'Generation Islamic State'.

A surveillance operation discovered two other close associates in the back of a lorry at Dover as they also tried to leave the country in November 2014.

Michael Coe and Simon Keeler, along with former boxer Anthony Small, were all cleared of attempting to travel to Syria last year, although Keeler was later arrested in Hungary.

Investigators discovered that Haque, using the name Abu Fulan, was in regular touch with Coe, 34, but were unable to retrieve the contents of the messages because Coe had downloaded a range of apps for communicating anonymously over the internet, including Telegram and WhatsApp.

Coe was Haque's gym partner and together they ran proselytising Dawah stalls in East Ham and Stratford in London, often wearing T-shirts with the logo 'Team Islam'.

All the men were key figures in the banned group al-Muhajiroun and its successor organisations, the latest of which is the Shariah Project.

Two other men from the group who also left the country and sought to join IS are now dead.

Abu Rahin Aziz, 32, one of a group of men involved in a fight with football fans in Oxford Street in May 2012, skipped bail and reappeared in Syria.

Aziz, from Luton, who had two children, travelled through Amsterdam to Syria from where he posted a picture of himself on the internet wearing military fatigues and brandishing an assault rifle in January 2015.

Using the name Abu Abdullah al-Britani, he boasted of how he had escaped from the authorities in Britain and encouraged others in the West to join IS or launch attacks at home.

Aziz was killed in a drone attack while in a car in Raqqa on July 6. His friend Mirza Ali, 38, a doctor from Chichester Wharf, Kent, who had been accused of participating in sectarian violence in London during another protest, also skipped bail and travelled to Croatia in an attempt to get to Syria.

He was deported from Croatia to his native Pakistan, where he joined a branch of the Pakistan Taliban.

Ali was killed in a CIA drone attack last year.