A video which emerged Friday shows Henning on his knees against a desert backdrop and wearing an orange prison-style outfit, with a masked militant standing over him wielding a combat knife before the video cuts to his beheading.
Henning, a 47-year-old from Manchester in northwest England who volunteered to help drive an aid convoy to Syria for Muslim charity Aid4Syria, was kidnapped 10 months ago.
Cameron said the fact Henning was captured and killed while trying to help others "demonstrates that there are no limits to the depravity of these ISIL terrorists."
He added that his "thoughts and prayers" were with Henning's wife Barbara, their children and friends.
The killing was also condemned by the Muslim Council of Britain, the country's biggest Muslim umbrella organisation.
Secretary general Shuja Shafi said it was a "despicable and offensive" act.
"Alan was a friend of Muslims, and he will be mourned by Muslims," he added.
"In this period of Hajj and this festival of Eid, Muslims remember the mercy of God and the emphasis God places on human life.
"Alan Henning's murderers have clearly gone against that spirit of Islam."
----------------------------------------------------------------------------------------------------------
Henning, a 47-year-old from Manchester in northwest England who volunteered to help drive an aid convoy to Syria for Muslim charity Aid4Syria, was kidnapped 10 months ago.
Cameron said the fact Henning was captured and killed while trying to help others "demonstrates that there are no limits to the depravity of these ISIL terrorists."
He added that his "thoughts and prayers" were with Henning's wife Barbara, their children and friends.
The killing was also condemned by the Muslim Council of Britain, the country's biggest Muslim umbrella organisation.
Secretary general Shuja Shafi said it was a "despicable and offensive" act.
"Alan was a friend of Muslims, and he will be mourned by Muslims," he added.
"In this period of Hajj and this festival of Eid, Muslims remember the mercy of God and the emphasis God places on human life.
"Alan Henning's murderers have clearly gone against that spirit of Islam."
----------------------------------------------------------------------------------------------------------