Is Cameron right, do some in the Muslim community 'quietly condone' IS ideology?
Speaking at a security conference in the Slovakian capital Bratislava later, he will also stress the importance of tackling radicalisation at its source.
Mr Cameron will highlight the role families and communities can play in countering such radicalisation.
He will say: "The cause is ideological. It is an Islamist extremist ideology, one that says the West is bad and democracy is wrong, that women are inferior and homosexuality is evil.
"It says religious doctrine trumps the rule of law and Caliphate trumps nation state and it justifies violence in asserting itself and achieving its aims. The question is: How do people arrive at this worldview?
"I am clear that one of the reasons is that there are people who hold some of these views who don't go as far as advocating violence, but do buy into some of these prejudices, giving the extreme Islamist narrative weight and telling fellow Muslims, 'You are part of this.'""