Prime minister David Cameron has said the "brutal murder" of Alan Henning was "completely unforgivable" and vowed to hunt down his Isis killers.
He made the comments at Chequers following a briefing with military and intelligence officials, including the head of MI5, on Saturday.
They came the morning after a video purportedly showing the beheading of the taxi driver, 47, from Eccles, Salford, was released by Islamic State.
Mr Cameron said: "The murder of Alan Henning is absolutely abhorrent.
"It is senseless, it is completely unforgiveable. Anyone in any doubt about this organisation can now see how truly repulsive and barbaric it is.
Married father-of-two Mr Henning was part of a convoy delivering much needed aid to Syria when he was captured on Boxing Day last December, shortly after crossing over the Turkish border into the country.
It was the third trip he had made to the Middle East with charity groups, the last with Worcester-based charity Al-Fatiha Global .
He was one of only eight aid workers to continue into Syria on the trip.
Islamic State militant "Jihadi John" can be seen in the video alongside Mr Henning, who delivered a message to the west before he was beheaded.
It follows the same pattern as videos showing the deaths of Americans James Foley and Steven Sotloff and Briton David Haines. Mr Henning appeared at the end of the video of aid worker Mr Haines.
US President Barack Obama and French President Francois Hollande are among world leaders to have condemned the killing of married father-of-two Mr Henning.
Mr Obama said: “Mr Henning worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom.
“Standing together with our UK friends and allies, we will work to bring the perpetrators of Alan’s murder – as well as the murders of Jim Foley, Steven Sotloff and David Haines – to justice.”
Mr Hollande said he was “outraged by the heinous crime” and vowed that it would not go unpunished.
This week Mr Henning’s wife, Barbara, made a desperate plea with his captors to release her husband.
His brother-in-law, Colin Livesey has described his murderers as "scum" and said he hoped they "get what's coming to them". He also said the British government could have done more to help him.
Yesterday Mr Cameron tweeted: “The brutal murder of Alan Henning by ISIL shows just how barbaric these terrorists are. My thoughts are with his wife and their children."
Labour leader Ed Miliband also took to Twitter, stating Mr Henning's murder was "appalling and barbaric". He added in a separate tweet: "Those who killed him have revealed only their lack of humanity and brutality."
Deputy Prime Minister Nick Clegg said the government was "resolved to defeat this evil" adding Mr Henning was a "man moved to selflessly help those in need".
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