The Islamic State has released a video claiming to show the beheading of British captive David Haines.
The father-of-two, an aid worker, was abducted by militants in Syria last year, while working for relief agency ACTED in Syria.
The video titled A Message to the Allies of America, begins with recent news footage of Prime Minister David Cameron discussing Britain's foreign policy towards Islamic State.
It then appears to show Mr Haines talking to the camera in the desert before being murdered.
Forced to condemn the UK government, the victim looks into the camera and says: "My name is David Cawthorne Haines. I would like to declare that I hold you David Cameron entirely responsible for my execution.
"You entered voluntarily into a coalition with the United States against the Islamic State just as your predecessor Tony Blair did, following a trend against our British prime ministers who can't find the courage to say 'no' to the Americans.
"Unfortunately it is we the British public that in the end will pay the price for our Parliament's selfish decisions."
The Islamic State militant, who has a British accent, adds: "This British man has to the pay the price for your promise Cameron to arm the Peshmerga against the Islamic State. Ironically he has spent a decade of his life serving under the same Royal Air Force that is responsible for delivering those arms.
"Your evil alliance with America which continues to strike the Muslims of Iraq and most recently bombed the Haditha Dam will only accelerate your destruction, and playing the role of the obedient lapdog, Cameron, will only drag you and your people into another bloody and unwinnable war."
The militant adds: "If you, Cameron, persist in fighting the Islamic State then you like your master Obama, will have the blood of your people on your hands."
The footage then shows what is believed to be the body of Mr Haines. Another hostage, believed to be British, is then paraded in front of the camera.
The Foreign Office has said it is working to verify the authenticity of the video. A statement released by the Foreign and Commonwealth said: "We are aware of the video. We are working urgently to verify the content. If true, this is another disgusting murder. We are offering the family every support possible. They have asked to be left alone at this time."
In the new video footage, which has been posted on sites such as YouTube, the extremists have also threatened to execute another Briton.
The Foreign and Commonwealth Office (FCO Islamic State (IS) had earlier confirmed that extremists had threatened to kill Mr Haines and had not responded to any of the family's attempts to make contact.
The release of the video comes hours after the family urged his captors to contact them.
David Cameron has reacted to the murder of David Haines saying it "is an act of pure evil and "we will do everything in our power to hunt down murderers. My heart goes out to his family who have shown extraordinary courage and fortitude."
Militants from IS have beheaded two American journalists, James Foley and Steven Sotloff, posting the evidence online in gruesome videos featuring a masked jihadist with a British accent dubbed 'Jihadi John'.
Mr Haines appeared at the end of a video showing the beheading of US journalist Steven Sotloff earlier this month.
The 44-year-old has a teenage daughter in Scotland from a previous marriage with wife Louise and a four-year-old daughter in Croatia from his present marriage.
He had been working for the French aid agency for just ten days when he was taken alongside Federico Motka, an Italian-Swiss aid worker freed four months ago after a ransom was paid.
Educated at Perth Academy secondary school, he has worked for aid agencies in some of the world's worst trouble spots.
He was in Libya during its civil war in 2011, working as head of mission for Handicap International which helps disabled people in poverty and conflict zones around the world.