Aid agency Acted said yesterday that the beheading of their employee David Haines by Islamic State (Isis) terrorists “must not remain unpunished.”

The former RAF engineer was volunteering with Acted when he was captured by the extremists in Syria in March 2013.

In a statement, Paris-based Acted said it was deeply appalled and horrified.

“The horrible assassination of David, an aid worker, goes against all humanitarian principles and is a crime against humanity,” it added.

“This barbaric crime must not remain unpunished.”

Isis released footage showing Mr Haines being butchered by a knife-wielding militant who appeared to speak with a British accent. 

The clip also included a threat to kill a second British hostage.

In a statement his brother, Mike Haines, said the father-of-two had been murdered “in cold blood.”

He remembered his brother’s “joy and anticipation” at the aid work he was looking forward to before he set off for Syria. 

Prime Minister David Cameron said Britain was ready to take “whatever steps are necessary” to help an international push to destroy the “evil” extremist group. 

He made an emotional press statement after meeting with the government’s emergency committee Cobra. 

Hailing Mr Haines as a “British hero,” Mr Cameron vowed to “hunt down those responsible and bring them to justice no matter how long it takes.”

Labour leader Ed Miliband added: “Acts like this will not weaken but strengthen the resolve of Britain and the international community to defeat Isis and their ideology.”