Footage uncovered by researchers at the International Centre for the Study of Radicalisation (ICSR) shows terrorist insurgents executing a prisoner, said to be a loyalist of President Bashar al-Assad.
The incident is thought to have happened in the last two weeks, near Raqqa in northern Syria, Sky news reported.
The footage was posted on the Instagram account of a man believed be from London.
The caption accompanying the video describes the prisoner as one of 'Bashar [Assad's] dogs' and says the execution was retribution for the deaths of four fellow militants.
The ICSR monitors the social media accounts of hundreds of foreign militants inside Syria.
They believe the man that posted the video is part of a group of British terrorists, known as Rahat Al Tawheed, an affiliate of the terrorist group ISIL, which controls large swathes of northern Syria.
"The execution of prisoners is a war crime in international law...it's incredibly serious" said Shiraz Maher, a senior ICSR researcher based at Kings College London
"We believe the main characters involved with Rahat al Tawheed come from London."
"We have deduced this based on our discussions with foreign fighters, our extensive record-keeping of foreign fighter activity in Syria, and our maintenance of social network maps which allows us to plot activity and associations in a visual form", Maher added.
Although no audible English is spoken in the execution video itself, analysts from the ICSR believe they have identified one of the gunmen as being a British citizen.
A man is seen in the video firing shots into the body of the prisoner in the seconds after the initial bullet was fired by the main executioner.
Analysis of the gunman's physical build, wrist watch and balaclava led the ICSR to conclude he is the same man seen speaking English with a London accent in other videos posted by the group.
"We don't know if the prisoner was alive or dead when he fired, but he did partake in the execution, he did fire shots at the individual from his weapon, and we believe he is a British citizen" said ICSR researcher Joseph Carter.
On two separate videos posted on YouTube, the man identified as the gunman by the ICSR is heard berating the British Muslim community for failing to provide sufficient financial support for the war, or the families the fighters have left behind.
The Government's counter-terrorism program last week launched a campaign urging the families of young men planning to travel to Syria to intervene.
It is estimated that up to 400 Britons have travelled to fight or train in Syria in the last two years.
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