The British Prime Minister David Cameron has vowed to hunt down the "barbaric and repulsive terrorists" responsible for beheading British aid worker Alan Henning.
Islamic State militants posted a video of his murder on the internet last night. Mr Hennis was a 47-year-old former taxi driver from Salford in England who was kidnapped last December in Syria by IS militants.
“My thoughts and prayers (tonight) are with Alan’s wife Barbara, their children and all those who loved him," said Mr Cameron.
“Alan had gone to Syria to help get aid to people of all faiths in their hour of need. The fact that he was taken hostage when trying to help others and now murdered demonstrates that there are no limits to the depravity of these Isil terrorists.
“We will do all we can to hunt down these murderers and bring them to justice.”
The British Foreign Office said it was offering his family every support possible.
Naved Siddiqi from the Islamic Society of Britain said he was disgusted by the killing.
"It was the way in which they were toying with the family, giving a glimmer of hope but all along they had this plan. They did it in such a brutal way, and also a very calculated way," he said.
French president Francois Hollande spoke of his “outrage” at the killing, US president Barack Obama condemned the “brutal murder”, and the United Nations Security Council described it as “heinous and cowardly”.
The footage shows Mr Henning kneeling beside a knifeman dressed in black in a desert setting.
It is the second such murder of a UK hostage after fellow aid worker David Haines was killed last month.
The gruesome video appears to show Mr Henning, who is dressed in orange, say: “I am Alan Henning. Because of our Parliament’s decision to attack the Islamic State, I, as a member of the British public, will now pay the price for that decision.”
The killer, who speaks with a British accent and is believed to be the man responsible for the previous beheadings, then makes a direct statement to the Prime Minister: “The blood of David Haines was on your hands, Cameron. Alan Henning will also be slaughtered, but his blood is on the hands of the British Parliament.”
The video then apparently shows the jihadist alongside an American hostage identified as Peter Kassig.
Addressing the US president, the militant says: “Obama, you have started your aerial bombardment in Sham (Syria), which keeps on striking our people. So it is only right that we continue to strike the necks of your people.”
Washington said the killing of Mr Henning “is again yet another just very clear example of the brutality of this group, and why the president has articulated and is moving out in a comprehensive way to degrade and destroy Isil”.
Lisa Monaco, assistant to the president for homeland security and counter-terrorism, said: “Our hearts go out to the British aid worker who we believe is in that video and to the remaining hostages and to their families.”
The latest video emerged a week after the House of Commons overwhelmingly backed the deployment of British military forces to bomb IS positions in Iraq.
The release of the footage came just hours after the father of British journalist John Cantlie, who is being held by IS, appealed for his son’s release.
Paul Cantlie said in a video from his hospital bed: “To those holding John: please know that he is a good man.
“He sought only to help the Syrian people and I ask you from all that is sacred to help us to allow him to return safely to those he loves and those who love him.”
In a statement, Mr Kassig’s parents Ed and Paula said: “The Kassig family extends our concern for the family of Alan Henning. We have read about his work and his generous character with great respect and admiration.
“We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe.”
The United Nations Security Council said: “This crime is a tragic reminder of the increasing dangers volunteer humanitarian personnel face every day in Syria.
“It also once again demonstrates the brutality of ISIL, which is responsible for thousands of abuses against the Syrian and Iraqi people.”
Two American journalists, James Foley and Steven Sotloff, were also beheaded by the terrorist organisation, which is also known as Isil.