A British press photojournalist who has been held prisoner by Islamic State (Isis) militants for almost two years has become the latest hostage to be forced to appear in an online video, this time delivering a propaganda message on behalf of the group in what he admits is a desperate gamble to save his life.

In a clip posted on the internet, John Cantlie calmly introduces himself and explains his predicament, before calling on the public to act to "change this seemingly inevitable sequence of events" by forcing the US and British governments to change their policies on hostage-taking.

The photojournalist also makes clear that he is under duress – and that he does not know whether he will succeed in his attempt to avoid being murdered.

"Now I know what you're thinking, you're thinking: 'He's only doing this because he's a prisoner. He's got a gun at his head and he's being forced to do this,' right?" he says.

"Well it's true I am a prisoner. That I cannot deny. But seeing as I've been abandoned by my government and my fate now lies in the hands of Islamic State I have nothing to lose. Maybe I will live and maybe I will die, but I want to take this opportunity to convey some facts that you can verify. Facts that if you contemplate might help preserving lives."

Cantlie makes clear that the three-and-a-half minute film is just the first of a series of "programmes" that Isis will be filming, and in which he will be appearing.

"I'm going to show you the truth as the western media tries to drag the public back to the abyss of another war with the Islamic State," he says.

An image of John Cantlie taken from the video. Photograph: Rex

On Thursday afternoon the 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."

Like Washington, the UK government has said it will not pay ransoms for the release of citizens by Isis or any other terrorist group.

The clip marks a new departure for Isis film-makers. The previous three videos, which have depicted the ritualistic murders of US journalists James Foley and Steven Sotloff and Scottish aid worker David Haines, and threatened the killing of aid convoy volunteer Alan Henning, have all followed the same format: each was shot out of doors, with the masked killer delivering messages in his distinctive London accent to Barack Obama and David Cameron.

The latest clip has been filmed indoors, is carefully lit, shot from two angles, and the British jihadist is not seen. The title that the group has given to it "Lend me your ears – messages from the British detainee John Cantlie" – quotes Shakespeare's Julius Caesar, but may also be a reference to the nickname that freed former hostages say they gave to British Isis jailors: the Beatles. Arabic subtitles appear at the foot of the screen.

Cantlie, 43, is pale, unshaven and thin, and traces of possible bruising can be seen under each eye. Like the three men whose murders were filmed, he is dressed in an orange jumpsuit. He appears composed, however, as he sits behind a desk and delivers his scripted statement directly to the camera. His performance appears to be well-rehearsed.

The video spread rapidly despite attempts by website providers to crack down on distribution. Numerous Twitter accounts which appear to be sympathetic to Isis used the hashtag #indyref and #ScotlandDecides to help boost circulation of the film.

Cantlie, a freelance photojournalist from Haslemere, Surrey, explains in the film that he used to work for some of the bigger newspapers and magazines in the UK including the Sunday Times, the Sun and the Sunday Telegraph.

He asks: "After two disastrous and hugely unpopular wars in Afghanistan and Iraq, why is it that our governments appear so keen to get involved in yet another unwinnable conflict? I'm going to show you the truth behind these systems and motivation of the Islamic State, and how the western media, the very organisation I used to work for, can twist and manipulate that truth to the public back home. There are two sides to every story – think you're getting the whole picture?

"And I'll show you the truth behind what happened when many European citizens were imprisoned and later released by the Islamic State, and how the British and American governments thought they could do it differently to every other European country. They negotiated with the Islamic state and got their people home while the British and Americans were left behind."

He goes on: "It's very alarming to see where this is all headed and it looks like history repeating itself yet again. There is time to change this seemingly inevitable sequence of events, but only if you, the public, act now. Join me for the next few programmes and I think you may be surprised by what you learn."

It is the second time that Cantlie has found himself in the hands of militants in Syria. He was rescued from kidnappers in 2012, but four months later chose to return to the country, where he was abducted a second time and sold on to Isis. While the photographer's first ordeal lasted just seven days, this time he has been held for 22 months, probably near Raqqa, the group's stronghold in the north of the country.

During his time in captivity, Cantlie has seen several fellow hostages set free after their governments agreed to pay millions of dollars in ransom. Cantlie is thought to have been abducted as he attempted to leave the country, along with Foley. The pair are understood to have been kidnapped after leaving an internet cafe in a town near the Turkish border At some point they were passed on to the Islamic StateIsis. .

No previous mention has been made in the media about Cantlie's plight at the request of the UK Foreign Office, which also advised his family and friends to say nothing in public about the abduction.

When Haines was first shown in an Isis video in September, the Foreign Office urged the media to show restraint, and not to report that two other British citizens – Cantlie and Henning – were also being held "because we assess that coverage will increase the threat to their lives".

Ten days later, the Foreign Office again asked that no mention be made of Cantlie when Henning, 47, was shown in the video that also depicted the murder of Haines.

Last night, British foreign secretary Phillip Hammond told reporters in Denmark that he was aware of the video but had not yet seen it.

Transcript of the Cantlie video

Hello my name is John Cantlie. I'm a British journalist who used to work for some of the bigger newspapers and magazines in the UK including the Sunday Times, the Sun and the Sunday Telegraph.

In November 2012 I came to Syria where I was subsequently captured by the Islamic State. Now nearly two years later many things have changed including the expansion of the Islamic State to include large areas of eastern Syria and western Iraq. A landmass bigger than Britain and many other nations.

Now I know what you're thinking, you're thinking: 'he's only doing this because he's a prisoner. He's got a gun at his head and he's being forced to do this,' right?

Well it's true I am a prisoner that I cannot deny but seeing as I've been abandoned by my government and my fate now lies in the hands of Islamic State I have nothing to lose.

Maybe I will live and maybe I will die, but I want to take this opportunity to convey some facts that you can verify. Facts that if you contemplate might help preserving lives.

Over the next few programmes, I'm going to show you the truth as the western media tries to drag the public back to the abyss of another war with the Islamic State.

After two disastrous and hugely unpopular wars in Afghanistan and Iraq, why is it that our governments appear so keen to get involved in yet another unwinnable conflict?

I'm going to show you the truth behind these systems and motivation of the Islamic State, and how the western media, the very organisation I used to work for can twist and manipulate that truth to the public back home.

There are two sides to every story – think you're getting the whole picture? And I'll show you the truth behind what happened when many European citizens were imprisoned and later released by the Islamic State and how the British and American governments thought they could do it differently to every other every other European country.

They negotiated with the Islamic state and got their people home whilse the British and Americans were left behind.

It's very alarming to see where this is all headed and it looks like history repeating itself yet again. There is time to change this seemingly inevitable sequence of events, but only if you, the public act now. Join me for the next few programmes and I think you may be surprised by what you learn.

• This article was amended on 19 September 2014. An earlier version said John Cantlie was from "Hazlemere in West Sussex,south of London".