The murder of David Haines was an “act of pure evil”, David Cameron has said after the release of a video appearing to show the UK hostage’s beheading.
The 44-year-old aid worker, from Perth, was kidnapped in Syria in 2013. He was being held by Islamic State militants who have already killed two Americans.
The latest video also includes a threat to kill a second British hostage.
The PM vowed to do everything possible to find the killers. Mr Haines’s family said he would be “missed terribly”.
‘Despicable and appalling’
In a statement released by the Foreign Office, Mike Haines said his brother, a father of two, “was and is loved by all his family”.
“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 said.
Mr Cameron, who is due to chair an emergency Cobra committee meeting later, said the murder of an innocent aid worker was “despicable and appalling”.
“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,” the prime minister added.
‘Grief and resolve’
The Foreign Office is working to verify the video, which was released on Saturday night. It begins with a clip of Mr Cameron and then features a man who appears to be Mr Haines dressed in orange overalls, kneeling in front of a masked man holding a knife.
The victim says: “My name is David Cawthorne Haines. I would like to declare that I hold you, David Cameron, entirely responsible for my execution.”
He says Mr Cameron had entered into a coalition with the US against the Islamic State “just as your predecessor Tony Blair did”.
“Unfortunately it is we the British public that in the end will pay the price for our parliament’s selfish decisions,” he said.
The militant, who appears to have a British accent, is then recorded as saying: “This British man has to the pay the price for your promise, Cameron, to arm the Peshmerga against the Islamic State.”
The UK has donated heavy machine guns and ammunition to authorities in Iraq to help fight IS militants, the Ministry of Defence previously said.
Kurdish forces, known as the Peshmerga, have been involved in heavy fighting with IS.
US President Barack Obama said “our hearts go out to the family of Mr Haines and to the people of the United Kingdom”.
In a statement he said the US would work with the UK and a “broad coalition of nations” to “bring the perpetrators of this outrageous act to justice”.
“The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve,” he added.
Scottish First Minister Alex Salmond said the release of the video “demonstrated a degree of brutality which defies description”.
Labour leader Ed Miliband said he was “sickened at the disgusting, barbaric killing” of Mr Haines.
‘Criminals and villains’
Senior UK imams and British Muslim community leaders have also condemned the killing.
“An attack on a British citizen is an attack on Britain and we raise our voices as a community united to deplore the actions of the terrorists Isis,” Dr Qari Asim, imam of the Makkah Mosque in Leeds said.
Sayed Ali Abbas Razawi, from Majlis-e-Ulama, which represents the majority of Shia Muslims in the UK and Europe, said militants were hiding behind a “false interpretation” of Islam, describing the group as “criminals and villains”.
Mr Haines was taken hostage in the village of Atmeh, in the Idlib province of Syria, in March 2013.
He had been helping French agency Acted deliver humanitarian aid, having previously helped local people in Libya and South Sudan.
The release of the video came hours after his family had made a direct appeal to IS to contact them.
IS – also known as Isis or Isil – has seized large swathes of territory in Iraq and Syria and declared a new caliphate – or Islamic state.
Militants from the extremist group have killed two US hostages in recent weeks, posting video evidence on the internet.
They threatened to kill Mr Haines during a video posted online showing the killing of US journalist Steven Sotloff earlier this month.
The extremist group also killed fellow US journalist, James Foley, last month.