ISIS beheads 3rd hostage, British aid worker… David Cameron declares war on “Jihadi John”

The Islamic State has released another video, this time showing the beheading of a British hostage and appears intended as a threat to countries backing military action in the Middle East.

The victim is aid worker David Haines, who went missing last year while working in Syria for an international aid agency.

The beheading is the third recorded and posted in the past month by Islamic State. It has similar visual characteristics to videos showing the beheadings of two American journalists, James Foley and Steven Sotloff.

News.com.au reported that the 2½-minute video titled "A Message to the Allies of America" includes what appears to be a threat directed at British Prime Minister David Cameron.

 "For being a lapdog, Cameron, you will drag your people into another bloody and unwinnable war," the video warns.

Cameron responded to the video by tweeting: "The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude. ... We will do everything in our power to hunt down these murderers and ensure they face justice, no matter how long it takes."

Cameron has vowed to bring Haines killers to justice as their second British hostage was today revealed as 47-year-old Alan Henning.

The British Islamic State executioner nicknamed 'Jihadi John' warned the taxi driver, from Greater Manchester, would be the next person to die after.

Mr Henning, nicknamed Gadget by colleagues who praised his sense of humour, was also an aid worker and is believed to have been working for a Muslim charity in a refugee camp.

Alan Henning at a Syrian refugee camp

Earlier today, David Haines' brother Mike quoted the Koran in an emotional on-camera tribute.

He told BBC: “My first reaction could be one of hatred. But my brother's life wasn't about hatred. It was about love for all men. We hoped, we prayed in our own way. Unfortunately it was not in our hands. It was in the hands of terrorists.

“I have become aware of a number of verses in the Koran that I feel are particularly apt at this time, if I may. Since good and evil cannot be equal, repel thou evil with something that is better. The Muslim faith is not to blame for Isil, nor is it the fault of people of Middle Eastern descent.

“The attraction of complete control and the use of terror as an implement of operational control has a widepsread appeal to many disenfranchised people throughout society, as you can see by the amount of foreign nationals - not just British - that are fighting for ISIL.

“My family and I agree with the government that we need to identify those travelling to fight with Isil and hold them responsible for their actions.”