British Muslim communities should take more responsibility for countering the threat of radicalisation, David Cameron will say today, as he warns of the dangers posed by people who "quietly condone" Islamic State's extremist ideology.

At a security conference in the Slovakian capital Bratislava, the Prime Minister will stress the importance of trying to stop radicalisation at its source and highlight the role of families and communities in tackling the problem.

The Prime Minister's comments follow reports of the death of Britain's youngest suicide bomber and fears that three sisters have taken their nine children to join extremists.

He will say the cases highlight how young people from Britain are in danger of sliding into violent extremism, encouraged by online propaganda and people who support some of Islamic State's extreme views.

" The cause is ideological," he will say. "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'.

"This paves the way for young people to turn simmering prejudice into murderous intent. To go from listening to firebrand preachers online to boarding a plane to Istanbul and travelling onward to join the jihadis."

Mr Cameron will say that the fact the extremist message is supported in some sections of society can help add to the appeal of IS, also known as Isil, for youngsters "angry at the world" or "looking for an identity".

He will say: "We've always had angry young men and women buying into supposedly revolutionary causes. This one is evil, is it contradictory, it is futile - but it is particularly potent today.

"I think part of the reason it's so potent is that it has been given this credence. So if you're a troubled boy who is angry at the world or a girl looking for an identity, for something to believe in and there's something that is quietly condoned online or perhaps even in parts of your local community then it's less of a leap to go from a British teenager to an Isi fighter or an Isil wife than it would be for someone who hasn't been exposed to these things."

Mr Cameron will accept that the police and intelligence agencies have a role in preventing people from travelling to Syria but will stress the importance of tackling the factors that lead would-be jihadis to attempt to join IS in the first place.

"Too often we hear the argument that radicalisation is the fault of someone else," he will say.

"That blame game is wrong - and it is dangerous. By accepting the finger pointing - whether it's at agencies or authorities - we are ignoring the fact that the radicalisation starts with the individual and we would be in danger of overlooking many of the ways we must try to stop it at the source.

"We need to treat the causes, not just the symptoms. Of course, we will do everything we can to help the police and intelligence agencies to stop people travelling to Syria. But we mustn't miss the point: they are not responsible for the fact that people have decided they want to go."

A Number 10 source said Mr Cameron was making the point that "of course Government has a part to play but so do communities and so do families too".

The source said: "Clearly there are lots of Muslims in the UK that play a vital role in our country. But here he is saying that in some communities there may be individuals who are condoning this.

"His point is we need to have a frank debate about the role that everyone has to play. Government definitely has a part to play but so do others."

The source added: "Where people have concerns they can seek out advice, they can talk to the authorities, they can get advice from the police.

"There are things people can do if they have concerns about what is going on in their community. We are going to do a much better job of protecting ourselves and protecting our young people if they are having those conversations."

People could raise concerns with community leaders, Imams, the police or teachers, the source suggested.

Mr Cameron will highlight work being done in the UK and European Union to tackle online prograpanda used by IS "to spread its warped worldview".

A British police officer will be seconded to a new EU internet referral unit working with the industry to remove jihadist content.

The Prime Minister's speech follows an appeal from Home Secretary Theresa May to would-be jihadists, telling them "do not travel".

At a counter-terrorism conference in London yesterday she said: "We know that Isil aspires to become a state in its own right, and that they don't just want to recruit young men, they also want to lure young women and families too by selling them promises about starting a new life.

"But these promises are false: Isil is bringing death and destruction to Syria and Iraq, and has made clear its brutality in the murder, rape and torture of men, women and children."