David Cameron has vowed the UK will "hunt down" the Islamic State militants responsible for the beheading of a British aid worker.
On Saturday night, footage showing the beheading of the British hostage David Haines was released by the Islamic State (Isis) – shortly after his family had appealed for the group to contact them to discuss his release.
The video shows Mr Haines, who was captured by militants in Syria in 2013, wearing an orange jumpsuit and kneeling in the desert while he reads a pre-prepared script. It later shows what appears to be the aid worker's body.
On Sunday, the British Prime Minister described Mr Haines as a "British hero", and praised the "extraordinary courage" of his family. He said the UK would "hunt down those responsible and bring them to justice, no matter how long it takes", adding: "They are not Muslims, they are monsters."
As with previous videos showing the beheading of US journalists James Foley and Steven Sotloff by Isis, it ends with a threat to kill another hostage - a second British national, who was today named as .
Having returned to Downing Street to chair a meeting of the Government's emergency committee Cobra, Mr Cameron insisted that the latest killing would not change Britain's strategy against Isis.
Speaking from Downing Street this morning, the Prime Minister said that "we have to confront this menace", Mr Cameron added that this will not involve direct British involvement in fighting. The UK is instead playing a supporting role to the US, providing transport and supplies.
"We cannot just walk on by if we are to keep this country safe," he said. "Step by step, we must drive back, dismantle and ultimately destroy Isil and what it stands for."
The video begins with footage of David Cameron talking about using the UK's "military prowess" to put pressure on the Islamic State – which he describes as an "appalling organisation".
It then cuts away to show Mr Haines kneeling next to a man who experts say appears to be the same militant who featured in the videos showing the deaths of Foley and Sotloff.
In his speech, which seems to have been made under duress, the hostage says: "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 that in the end will pay the price for our Parliament's selfish decisions."
The Isis militant then speaks, and at one point mentions the bombing of the Haditha Dam by the US, an incident that only occurred in the past week, showing that the killing was carried out very recently.
He also addresses Mr Cameron directly, saying the Prime Minister will "pay the price for [his] promise to arm the Peshmerga (Kurdish military) against the Islamic State".
The militant finishes speaking and goes to hold a knife to Mr Haines' throat, before the video cuts away to show what appears to be his body.
The US President, Barack Obama, has led a host of international leaders in condemning the latest high-profile murder by Isis.
Mr Obama, who was himself addressed in the Foley and Sotloff videos, has recently outlined a battle plan to bring increased military action against Isis in Syria.
In a statement, he said: "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.
In a statement, Mr Abbott said Mr Haines's killing was "further demonstration that this particular terrorist group does not just do evil, but exalts in doing evil".
Mr Obama also reiterated his support for the creation of a "coalition of nations" to deal with Isis, which he said would "bring the perpetrators of this outrageous act to justice".
Tony Abbott, the Australian prime minister, said on Sunday that his country would also be scaling up its military effort against Isis, contributing a further 600 troops and up to 10 military aircraft.
In Britain the murder was condemned by the Labour leader Ed Miliband and the Deputy Prime Minister Nick Clegg, while First Minister Alex Salmond has called a meeting of the Scottish equivalent to Cobra.
Mr Haines grew up in Perth, Scotland. A 44-year-old father of two, he worked for the Royal Mail and then the Royal Air Force as an aircraft engineer.
He worked for aid agencies in some of the world's harshest combat zones in recent years, including Libya during the 2011 civil war. He was taken hostage while working for the agency ACTED in March 2013, as he worked to bring aid to the Syrian people.
Mike Haines, David's brother, said in a statement that the aid worker had been murdered in cold blood.
He said: "David was like so very many of us, just another bloke. 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."