The British aid volunteer Alan Henning, held hostage by ISIL in Syria, is said to have been beheaded.
A video has been released by the extremist group purporting to show his execution.
The killing was reported on the website of the SITE Intelligence Group, which monitors global terrorism.
The 47-year-old Salford taxi driver, who went to Syria with Muslim friends to deliver aid to refugees, is shown in the same pose as other victims in previous videos – kneeling next to a masked militant.
In the video Alan Henning’s death is blamed on what is called “the British parliament’s decision to attack Islamic State”.
ISIL had threatened to kill the Briton in a previous video showing the execution of another British hostage, David Haines.
The apparent murder of the taxi driver has been roundly condemned.
Prime Minister David Cameron said in a statement: “The brutal murder of Alan Henning by ISIL shows just how barbaric and repulsive these terrorists are. My thoughts and prayers tonight are with Alan’s wife Barbara, their children and all those who loved him.
“Alan had gone to Syria to help get aid to people of all faiths in their hour of need. We will do all we can to hunt down these murderers and bring them to justice.”
The White House said if the video was authentic it was another example of ISIL’s brutality.
The Muslim Council of Britain called the reported murder "a despicable act". Many Muslims said his murder would be against Islamic law.
Alan Henning’s wife had appealed for his release, stressing his innocence – apparently to no avail.
News of the killing comes after British fighter planes joined the US and its Arab allies – Jordan, Saudi Arabia and the United Arab Emirates – in attacking ISIL in Iraq.
RAF jets struck at targets days after the parliamentary vote.
Alan Henning is the fifth western hostage to be beheaded by ISIL or a related group.
Ominously, the latest video also shows a captive American aid worker who is now threatened.