Warning: David Cameron will will urge families to speak out against the ‘poisonous ideology’ driving hundreds of young people to wage jihad for ISIS

Too many British Muslims ‘quietly condone’ extremism, David Cameron will say today.

In a blunt speech, he will urge families to speak out against the ‘poisonous ideology’ driving hundreds of young people to wage jihad for Islamic State.

The Prime Minister will also call for parents to stop blaming police and the security services for failing to prevent British teenagers heading to Syria.

Parts of the Muslim community are guilty of normalising hatred of democracy and western values, he warns – making it easier for violent extremism to take hold.

At the weekend, 17-year-old Talha Asmal became Britain’s youngest suicide bomber when he detonated a car loaded with explosives in the northern Iraqi town of Baiji.

Speaking at a major security conference in Slovakia, Mr Cameron will say: ‘We’ve always had angry young men and women buying into supposedly revolutionary causes. This one is evil, it is contradictory, it is futile.’

But he will point out it is easier to go from being a troubled teenager to a jihadist if extremism ‘is quietly condoned online or perhaps even in parts of your local community’.

Islamic State and its medieval outlook are ‘one of the biggest threats our world has faced’, the Prime Minister will add.

The speech, made the day after the start of the holy festival of Ramadan, is likely to spark anger in parts of the Muslim community.

But Downing Street said Mr Cameron believed parents must speak out.

A source said people should ‘look at the radicalisation of people here in the UK and what is going on in the areas where they live, their households and their communities’.

‘People should voice their concerns,’ the source said. ‘There might be more senior people in their community they want to talk to, or they can talk to the authorities or the police.’

Mr Cameron’s intervention comes amid mounting concern about the radicalisation of young British Muslims.

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Concerns: On Monday, it emerged that three British sisters and their nine children had travelled to Syria

On Monday, it emerged that three British sisters from Bradford and their nine children had travelled to join IS in Syria.

Suicide bomber Asmal’s family, from Dewsbury, also in West Yorkshire, said they were left ‘utterly devastated and heartbroken’.

Mr Cameron will say the cases are part of a grave trend that threatens Britain: ‘These are young people, boys and girls, leaving often loving, well-to-do homes, good schools and bright prospects travelling thousands of miles from home to strap explosives to their chests and blow themselves up and kill innocent people.

Suicide bomber: 17-year-old Briton Talha Asmal

'To live in a place where marriage is legal at nine and where women’s role is to serve the jihadists, to be part of a so-called state whose fanatics are plotting and encouraging acts of despicable terrorism in the countries from which they have come.’

The Prime Minister will further warn that they are driven by an extremist ideology, parts of which have at least a foothold in ordinary British Muslim communities.

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 world view?

‘One of the reasons is 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 to join the jihadis.’

Mr Cameron will hit out at those in the Muslim community who have blamed the security services for failing to stop the estimated 700 youngsters who have travelled to join IS in recent months. ‘Too often we hear the argument that radicalisation is the fault of someone else,’ he will say.

Today’s speech has echoes of a 2011 address in which Mr Cameron blamed multiculturalism for allowing the rise of extremism in Britain.