Isis have released a video purporting to show the execution of British aid worker David Haines.
Prime Minister David Cameron 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."
Mike Haines, the brother of David Haines, said in a statement in the early hours of Sunday that his brother had been murdered in cold blood. He added: “David was like so very many of us, just another bloke. Born in 1970 to parents who loved us both, our childhood was centred around our family. Holidays in caravans and tents, days away as a family which we remember fondly. David and I were brought up to know right from wrong, although we might not with the innocence of youth have always chosen right. David was a good brother, there when I needed him and absent when I didn’t. I hope that he felt the same way about me. He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass. He would probably say the same about me.
“After leaving school he worked with the Royal Mail before joining the RAF as an aircraft engineer. He married his childhood sweetheart Louise and in the due process of time had a wee lass Bethany. He was - and no doubt wherever he is - exceptionally proud of Bethany.
“David served with the UN in the Balkans, helping people in real need. There are many accolades from people in that region that David helped. He helped whoever needed help, regardless of race, creed or religion. During this time David began to decide that humanitarian work was the field he wanted to work in.
“David left the RAF and was employed by Scotrail. As with every job, David entered into it with enthusiasm. David met and married his second wife Dragana and they have a four year old daughter Athea.
“David was most alive and enthusiastic in his humanitarian roles. His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair.
“He was and is loved by all his family and will be missed terribly.”
Prior to the Prime Minister's statement, Foreign Office staff issued a statement that they were urgently trying to verify the authenticity of the film.
Mr Cameron came back to Downing Street last night and said he would be attending an emergency COBRA meeting on Sunday morning.
Mr Haines, a father of two, was abducted while working for a non-governmental organisation in Syria in March 2013.
Labour leader Ed Miliband said: "I am sickened at the disgusting, barbaric killing of David Haines.
"He was somebody whose only purpose was to help innocent people, themselves victims of conflict."
Mr Miliband continued: "My deepest condolences and thoughts are with his family as they cope with this terrible crime. And the hearts of the British people will go out to them."
The film was titled “Message To The Allies of America” and appeared to show Mr Haines, who Isis threatened to kill in a previous video, kneeling in the desert.
Speaking to the camera, the hostage identified by Isis as Mr Haines, 44, says: “I would like to declare that I hold you David Cameron entirely responsible for my execution.”
He later adds: “Unfortunately it is we, the British public, who will pay the price for our parliament’s selfish decisions.”
Just as in the earlier videos released by Isis which showed the beheadings of American journalists James Foley and Steven Sotloff in recent weeks, a man clad all in black speaks in the film before Mr Haines is apparently beheaded.
The man blames the United Kingdom for its involvement with the United States in the fightback against Isis in Iraq and Syria.
He says in the film: “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.”
Isis have captured large swathes of Iraq and Syria and have urged Muslims from around the world to join them.
A father of two daughters, he was living in Croatia before his kidnapping. He had previously worked in some of the world’s most dangerous regions, in his role as an aid worker, including Libya and South Sudan.
The video was released after a day in which Mr Haines’s family had made an attempt to contact his captors – although a statement released by the Foreign Office said that their requests for talks had been ignored. Foreign Secretary Philip Hammond had said the government was doing “everything” it could to protect him.
Mr Foley’s death was reported a month ago when Isis released its first “Message To America” video on the internet. Two weeks later Steven Sotloff was beheaded in a similar film. At the end of that second video, Mr Haines was shown in an orange jumpsuit and threatened with death.
The Foreign Office said last night it was working to verify the video.
