A shocking video released online last night by the terrorist group appeared to show the beheading of Mr Haines, 44, from Perth, who was kidnapped in Syria in March last year.
In a terrifying repeat of the videoed beheadings of US journalists Steven Sotloff and James Foley, at the end of the footage the group threatened to kill another British hostage.
Prime Minister David Cameron has rushed back to Downing Street, branding the apparent murder as an act of “pure evil” as he prepares to chair a meeting of the emergency Cobra committee.
IS had threatened to kill father-of-two Mr Haines, who last month appeared at the end of footage showing the execution of a US journalist, if the British and American governments did not halt military action.
The new graphic video shows Mr Haines forced to read a political statement while handcuffed and kneeling on the ground next to a knifeman believed to be Jihadi John.
The Foreign Office is urgently looking in to the video. A spokesman said: “If true, this is another disgusting murder. We are aware of the video and are working urgently to verify the contents.”
Prime Minister David Cameron said: “This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.
David holding his four-year-old daughter [NC]
This is a terrible event and a further indication as to the fact that we should no longer be dithering about our reaction to IS
Colonel Richard Kemp, former Commander of British Forces in Afghanistan
“My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal.
“We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes.”
In the video, entitled ‘Message to Allies of America’ the militant, who has a British accent, says: “This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State.
“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 continues.
The footage appears to show the man take a knife to Mr Haines’ throat, before a body and head are shown.
In a sinister warning to the British government, the militant says: “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.”
news, world, syria, ISIS, gun, violence, david haines, beheading, muslim, children, social media, jihadi johnDavid Cameron arrived at Downing St in the early morning and will chair an emergency Cobra meeting [Reuters]
Colonel Richard Kemp, former Commander of British Forces in Afghanistan, last night said: “Having worked in British Government on hostage rescue I know how much effort has gone in to saving David Haines. Now is the time for retribution.
“This is a terrible event and a further indication as to the fact that we should no longer be dithering about our reaction to IS.
“This tragedy is the reason, if one more were needed, to attack IS with everything we’ve got in both Iraq and Syria, from the air and if necessary from the ground. We should do all we can to enable local forces to deal with IS but we cannot depend on that – we need to be ready to send Armed Forces in on the ground to assist them.”
The news came hours after the family of Mr Haines issued a plea to his captors to contact them. IS had ignored all of their attempts at contact.
Intelligence officials fear Jihadi John and hundreds of others who have left Britain to join IS in Iraq and Syria were radicalised in young offenders institutions.
Mr Haines was taken hostage while working in Syria in March 2013, having previously helped people in Libya and South Sudan.