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Islamic State militants have released footage claiming to show the beheading of British hostage David Haines.

The aid worker, 44, was captured by ISIS in Syria in March 2013.

The plight of the dad-of-two was revealed when he appeared at the end of a video showing the beheading of US journalist Steven Sotloff earlier this month.

ISIS had threatened to kill Mr Haines if world leaders do not bow to their demands.

Tonight, the extremists posted a new video lasting 2minutes and 30 seconds which appears to show the beheading of David Haines.

The Prime Minister rushed back to Downing Street from a society wedding in Hampshire and will chair an emergency meeting of COBRA on Sunday morning.

He condemned the murder of David Haines as "an act of pure evil".

The Prime Minister said: "This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.

"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."

He was joined by the the leader of the opposition Ed Miliband who said: "I am sickened at the disgusting, barbaric killing of David Haines."

President Barack Obama also condemned the murder and promised the United States would "destroy" ISIS.

President Obama said in a statement: "The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL (ISIS).

"Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom.

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve.

"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world."

The new execution video begins with a news clip of David Cameron talking about Britain's policy towards "this appalling organisation".

This fades out before text appears which reads: "A message to the allies of America".

The video then shows a man, believed to be David Haines, on his knees in an orange jumpsuit with his hands handcuffed behind his back.

An Islamic militant - believed to be the infamous Jihadi John - stands behind him holding a knife.

The man believed to be David Haines speaks first, saying: "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 amongst our British Prime Minsters who can't find the courage to say no to the Americans'

"Unfortunately it is we the British public who will pay the price for our parliament's selfish decisions."

The man believed to be Jihadi John then says: "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 un-winable war.

The footage then shows what is believed to be the dead body of Mr Haines.

Another British hostage is then paraded to the camera.

Jihadi John 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."

Before tonight's video, ISIS militants had not responded to attempts made by David Haines' family to make contact.

Mr Haines was born in East Yorkshire but raised in Perthshire, and before his capture was living in Croatia with his second wife Dragana.

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He has a 17-year-old daughter with his first wife Louise, and a four-year-old daughter with Dragana who described him as a "fantastic man and father".

Mr Haines was taken while working for ACTED, having previously helped people in Libya and South Sudan.

Militants from IS have beheaded two American journalists, Jim Foley and Sotloff, posting the evidence online in gruesome videos featuring a masked jihadist with a British accent.

Foreign Secretary Philip Hammond has said the FCO will be doing "everything" they could to protect Mr Haines.

Educated at Perth Academy secondary school, he has worked for aid agencies in some of the world's worst trouble spots, including Libya and South Sudan.

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.