Hostage David Haines's murder condemned by Cameron
14 September 2014 Last updated at 22:33 BST
The prime minister has condemned the murder of the British aid worker, David Haines, by Islamic State militants, as an act of pure evil.
David Cameron has vowed to do everything possible to hunt down his killers.
The video of Mr Haines's killing also includes a threat to kill a second British hostage, who has been named as Alan Henning, 47.
Caroline Hawley reports.