The video shows a man sitting at a desk, looking calm and reading from a prepared script, promising to tell the “truth” about the self-proclaimed Islamic State (IS) movement.
The video is the latest footage showing a British hostage to emerge from IS militants, following the brutal murder of David Haines and the threat to kill aid worker Alan Henning.
The man in the video, who is not Mr Henning, said he was a prisoner and his “fate now lies in the hands of the Islamic State”.
The footage, which appears to be well produced with good quality sound and lighting, shows the man dressed in the now familiar orange outfit worn by other hostages in IS videos.
Foreign Secretary Phillip Hammond told reporters in Denmark that he was aware of the video but had not yet seen it.
He said: “I’ve heard the reports of another video being released on social media.
“These videos can be very distressing for the families of the individuals involved.
“But I haven’t seen the video myself, as you’ll appreciate, I’ve been in meetings here all day, but obviously we’ll look very closely at any material that has been released on the internet.”
Asked if he had any news on Mr Henning, the Foreign Secretary said: “I have nothing to add on the situation of the hostages.
“Obviously we are doing everything we can to support them and their families, but you wouldn’t expect me to give a running commentary.”
And asked if he knew where the hostages are, he said “As I say, you wouldn’t expect me to give a running commentary, I think.”
In the video, the prisoner said: “Now, I know what you are thinking, you are thinking ‘he is only doing this because he is a prisoner, he’s got a gun at his head and he is being forced to do this’, right?
“Well, it’s true, I am a prisoner, that I cannot deny. But, seeing as I have been abandoned by my government and my fate now lies in the hands of the Islamic State, I have nothing to lose.”
He promises that “over the next few programmes” he will show the “truth as the Western media tries to drag the public back to the abyss of another war with the Islamic State”.