Video has emerged showing what is believed to be a British jihadist fighter involved in a chilling execution of a prisoner loyal to President Bashar Assad in northern Syria.
The footage was posted on the Instagram account of a man understood to be part of a British extremist group and refers to the victim as one of "Bashar's dogs" who, the account claims, confessed to killing four men and raping a woman.
The brutal footage shows the doomed man bound and kneeling before he is shot through the back of the head with a pistol by a man whose upper body remains off camera.
A second man then fires repeated shots into the prisoner's body with a machine gun as the camera pans back to show him wearing a balaclava.
The camera briefly glimpses two other jihadist fighters surrounding the body. The killing is reported to have taken place in the past few weeks in Raqqa, northern Syria.
The British man is suspected to be the second gunman, who fires the machine gun.
His build and clothing, in particular his chunky black watch and head mask, appear to mark him out as a man who features in several other extremist videos posted online by the group known as Rayat al Tawheed, who are prolific across social media.
A spokesperson for the International Centre for the Study of Radicalisation (ICSR), who closely monitor the group's output, told ITV News Online it is the first execution they have seen in which a British citizen is believed to be involved.
ICSR researcher Joseph Carter said: "From what we've looked at we do believe he's a British-born fighter.
"He's one of four British men in Rahat al Tawheed who we see appearing in their videos and one of the two - both bigger guys - who heavily promote (their cause)."
The group, which is associated with the dominant group of Islamic fighters ISIS in northern Syria, use their online profile to distribute their extremist messages.
Speaking in British accents, the men appear in videos glorifying violence, urging western recruitment and calling for donations towards firearms.
"Rahat al Tawheed is specifically targeting young westerners with their videos and their graphics on social media," Mr Carter added.
The group also recently posted a video online in which a British man in a balaclava showed the apparent living conditions of the rebel fighters.
While rejecting claims that the jihadists have been living in luxury, the clip showed an Apple laptop computer and western products like Tabasco sauce, body scrub and hair products which the men brought with them to the conflict area.