Speaking in the House of Commons, Cameron stressed that the UK will not "waver" in its aim to defeat terrorism and that every possible option will be considered to protect the British man even as he condemned the "barbaric" beheading of a second US hostage.
"There is no doubt in my mind that the many tens of millions of dollars that ISIL (IS) have raised from ransom payments is going into promoting terrorism including terrorism affecting our own country," he said.
The British hostage has family in Scotland but relatives do not want him named.
An IS video posted online two weeks ago showed the killing of another US journalist, James Foley.
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The British hostage appears at the end of the latest video, released yesterday and titled 'A Second Message To America'.
The US has confirmed the video is authentic.
Asked if Britain will try to "extricate" the British hostage, UK Foreign Secretary Philip Hammond referred to the previous failed attempt to rescue him.