The family of Alan Henning - the British aid worker killed by Islamic State militants - say they are 'numb with grief' and devastated by the news of his death.

In a statement released this afternoon, the family have described him as a 'decent, caring human being' and say they are 'extremely proud' of what he achieved.

"There are few words to describe how we feel at this moment. Myself, Lucy and Adam, and all of Alan's family and friends are numb with grief."

"Alan was a decent, caring human being. His interest was in the welfare of others. He will be remembered for this and we as a family are extremely proud of him and what he achieved and the people he helped,” they added.

The British Prime Minister has vowed to do everything possible to "hunt down" his killers, and bring them to justice.

Muslims living across Britain have joined David Cameron's condemnation of the 47-year-old's murder by the terrorist group.

The video of his beheading was posted online last night and follows British airstrikes in Iraq to defeat the extremists.

Speaking at Manchester Central Mosque, Imam Asim Hussain says Alan Henning's killers must be brought to justice.

“The killing of Alan Henning was a cowardly and criminal act of appalling brutality by a group who not represent Islam at all and in fact are an insult to the Islamic faith. The killing of Alan Henning marks the end of this group,” Hussain said.