The British Prime Minister says the murder of British hostage Alan Henning by Islamic State militants is "aborrent" and "unforgiveable".
David Cameron has promised to hunt down the terrorists after jihadists posted a video of him being beheaded online.
The 47 year old taxi driver was delivering aid to Syria in December when he was kidnapped.
Mr Cameron says everything will be done to defeat the terror group.
Meanwhile, Muslims across Britain are condemning the killing of a second British hostage by Islamic State.
The former Secretary-General of the Muslim Council of Britain, Sir Iqbal Sacranie, says the militants do not represent Islam.
