In an eight-second excerpt the camera pans away from a flatbed truck upon which a grenade-launcher is mounted, to reveal a hooded Emwazi in profile.
Swathed in black and wearing a cap under his hood, he then turns and stares directly into the lens before looking away to scan the desert.
No sound accompanies the footage but the Mail on Sunday says it has been played an audio excerpt from the full video.
In it, the man thought to be Jihadi John is heard clearly saying, in a British accent: "I will carry on cutting heads."
The film was secretly obtained by rebel fighters of the Free Syrian Army, who sent it to one of their colleagues, known as Abu Rashid, in the Bulgarian capital of Sofia.
It was then shown in full to another FSA-linked activist in Istanbul who said: "He looks at the camera and says, "I am Mohammed Emwazi. I will soon go back to Britain with the Khalifa" [the leader of Isis]."
The activist continues: "He said something more about cutting heads off, 'We will kill the kuffar.' Then the camera shows two masked men that looked like bodyguards."
The Mail on Sunday showed the clip to one of the UK's leading experts on facial mapping, a former policeman who regularly acts as an expert witness in court cases.
After comparing the film to existing images of Emwazi, the expert said: "I have noted a number of apparent similarities in both the morphological and the proportional comparisons and in my opinion the images lend support to the contention that they are the same person."
This, he added, would be sufficient to result in "an arrest being made in most cases and may get a conviction in court".
Abu Rashid, from the Syrian city of Aleppo, is believed to have received the grainy footage on his smart phone through the video messaging app called Viber. He later passed a copy of the video to Bulgarian counter-terrorist police.
After Emwazi's true identity was revealed, various pictures of him emerged from his school days in the UK. In one, taken from a Westminster University student card, he is seen wearing a baseball cap.
Last month there was speculation that Emwazi had fled with Isis to Libya. Other reports, however, suggested he was on the run inside Syria from his fellow fighters, who supposedly wanted him dead. The new video appears to contradict these reports.
Jihadi John is one of the world's most wanted men with a £6 million bounty placed on his head by the US. He became the world's most infamous jihadi after he beheaded American captive James Foley, 40, in the Syrian desert a year ago.
It was the first in a series of bloody executions of several Western hostages over the next five months.
After Foley, fellow American Steven Sotloff, 31, was beheaded and Briton David Haines, 44, from Perth, Scotland, became Jihadi John's third victim. Haines's death was followed by that of Manchester taxi-driver Alan Henning, 47, whose execution provoked condemnation from the West and the Islamic world as he had travelled to Syria to do humanitarian work with a group of British Muslims.
American aid worker Peter Kassig, 26, was Jihadi John's fifth Western victim and, in January, Japanese hostages Haruna Yukawa, 42, and Goto, 47, became his last two.
Jihadi John has also been seen in a video taking part in the beheadings of at least 17 Syrian soldiers.
British journalist John Cantlie, 45, is the last known Western hostage being held by Isis. He featured in Isis propaganda videos last year, when he was shown in the northern Iraqi city of Mosul and the Syrian city of Aleppo.
He wrote an article in May in Isis' monthly magazine called Dabiq, warning how easy it was for the group to acquire nuclear weapons.
- Mail on Sunday
