A video purporting to show UK hostage Alan Henning being beheaded has been released by Islamic State militants, according to the BBC.
The Salford taxi driver was delivering aid to Syria in December when he was kidnapped and then held hostage by IS.
IS threatened to kill him in footage last month showing the death of Briton David Haines, and in this video they threaten US aid worker Peter Kassig.
David Cameron said Britain would do all it could “to hunt down these murderers and bring them to justice”.
The prime minister said the killing of father-of-two, Mr Henning, 47, showed “how barbaric and repulsive” IS was.
“My thoughts and prayers tonight are with Alan’s wife Barbara, their children and all those who loved him,” he said.
“Alan had gone to Syria to help get aid to people of all faiths in their hour of need.”
Mr Henning’s wife Barbara had this week appealed for her husband’s release, saying: “He is innocent.”
IS has previously released videos showing the apparent beheadings of two US journalists James Foley and Steven Sotloff, and British aid worker Mr Haines.
The video released on Friday is yet to be verified, but it appears to show Mr Henning kneeling beside a militant, dressed in black, in a desert setting.
The footage ends with an IS fighter threatening a man they identify as Mr Kassig.
In a statement, Mr Kassig’s family said he had converted to Islam and referred to him as Abdul Rahman Kassig.
They asked people around the world to pray for his release and that of “all innocent people being held hostage in the Middle East and around the globe”.
They also asked people to pray for Mr Henning’s family, adding: “We have read about his work and his generous character with great respect and admiration.”
The UK Foreign Office said in a statement: “We are aware of the video and are working urgently to verify the contents.
“If true, this is a further disgusting murder.
“We are offering the family every support possible; they ask to be left alone at this time.”
The US said it was evaluating the video and if proved real, it was “another demonstration of the brutality” of the militant group.