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Mr. Henning appears kneeling in the desert, in orange, shapeless clothes, as the executioner stands behind him and claims David Cameron “will have the blood of your people on your hands” if he joins an American alliance to fight the militants.

Mr. Henning is believed to have been seized when fighters loyal to ISIS overran the town of Ad Dana, where he was working with refugees.

He was thrown into a makeshift prison, and initially thought his work for a Muslim charity would save him.

A Syrian activist told a reporter earlier this year that he had spent a night in the same cell, and Mr. Henning had seemed in good spirits, believing he was about to be freed.

However, the activist, who later escaped, found Mr. Henning had later been moved to Raqqa, the city considered the capital of ISIS.

Friends of Mr. Henning, from Eccles, described him as a “lovely guy” who had been moved to act by the plight of children caught in the war.

A close friend said: “He was taking over old ambulances, just helping out as much as he could.

“They were supposed to be over there for about six months but he was kidnapped just a few days after he left.”

His wife, Barbara, had been told he was being well treated. The friend said: “He had his own mattress because the militant guys knew he was over there doing good.

“He started taking all this Syria stuff seriously when the war started. It was the fourth time he’d gone out there. He had a tattoo saying Aid 4 Syria on his arm. I think he thought it was his mission to help the kids out there.”