The UK’s prime minister hailed David Haines as a “British hero” and vowed to “hunt down those responsible and bring them to justice no matter how long it takes” after Islamic State (IS) posted a video of the beheading of the father of two.
A second British hostage, named as Alan Henning, was also shown, with a threat that he would be the next to be killed.
The killing appeared to have been carried out by the same British-accented jihadist responsible for the deaths of two American journalists held by the group, which is also known as Isil (Islamic State of Iraq and the Levant).
In an emotional statement after chairing a meeting of the Government’s Cobra emergency committee to discuss the killing, Mr Cameron said the brutal crime would “strengthen our resolve” to take on the threat posed by the Islamic extremists at home and abroad.
British people would be “sickened” that a fellow citizen could have carried out such an act, Mr Cameron said, and urged action to drain from society the “poison” of radicalisation — insisting IS and its adherents “are not Muslims, they are monsters”.
He remained tight-lipped on Britain’s role in the planned escalation of the military offensive in Iraq and Syria being co-ordinated by the US, and officials made clear there were no immediate plans to recall parliament to discuss the use of British armed forces.
But while he ruled out sending ground troops, Mr Cameron left the door open to air strikes — a course being urged on him by former military chiefs and some MPs.
“The murder of David Haines at the hands of Isil will not leave Britain to shirk our responsibility with our allies to deal with the threat that this organisation poses. It must strengthen our resolve,” he declared.
“Ultimately our security as a nation, the way we go about our everyday lives in this free and tolerant society that is Britain, has always depended on our readiness to act against those who stand for hatred and who stand for destruction. That is exactly what we will do.”
Fears for Mr Haines’ life had intensified when he was paraded as the next intended victim in the video of the death of journalist Steven Sotloff earlier this month and Mr Cameron paid tribute to the “extraordinary courage” shown by the Haines family since he was snatched in March last year.
David Haines’ brother said radicalisation presents a threat “to the wholesale safety of every person in the world”.
Mike Haines said harsh penalties should await Britons fleeing to Syria to join fundamentalists.
In a statement, Mr Haines said: “Our brother, son, father, nephew, husband and friend was brutally and cold-bloodedly murdered by ISIL after being held hostage since March 2013.
“We agree with the Government in that ISIL are extremely dangerous, and pose a threat to every nation, every religion, every politics and every person.
The militant who appears in the David Haines beheading video seems similar to the man who featured in recent clips of US hostages being murdered, a security expert has said.
Raffaello Pantucci, senior research fellow at defence think- tank the Royal United Services Institute, said there were numerous similarities in the films showing the killings of British aid worker Mr Haines and Americans James Foley and Steven Sotloff.
All three open with a media clip of the leader of the hostage’s home country — Barack Obama in the first two and David Cameron in the video featuring Mr Haines — before the captive reads what appears to be a prepared message.
In each video the militant is dressed in a similar all-black outfit with only his eyes visible, while all three hostages are seen kneeling in orange boiler suits in what appears to be a desert location.
He speaks with what appears to be a British accent.
The apparent killer stands to the left in all of the videos and holds a knife in his left hand.
Mr Pantucci said: “They are posed in a very similar way, opening with a message from Obama or Cameron. Then there is the similarity of having the hostage make a false statement to the camera.”
He said he was “pretty sure” the same Islamic State fighter appeared in all three videos.
“The voice sounds the same,” he said.
“He is clearly dressed in the same manner and his stature looks the same. It looks like the same figure.”