Family breakdown is being blamed for Britain's social problems in a new study.

A report by former Tory leader Iain Duncan Smith paints a grim picture of a "growing underclass in the UK" which has emerged as a result of "a catalogue of policy errors and omissions"

The report claims that family breakdown costs the nation £20 billion a year and fuels a violent crime wave which is tearing communities apart.

Mr Duncan Smith is calling for a "radical appraisal" of marriage policy in a bid to reduce social tensions.

He warned that policies such as tax credits and drug harm reduction strategies were behind high levels of social breakdown that "threaten the well-being not just of the underclass but of middle-class people living in once tranquil neighbourhoods."

Mr Duncan Smith said: "We see now a growing underclass of people who, actually in some senses, give up hope of rising above it."

He said he had been "shaken" to find that half of all unmarried couples with children broke up before the youngster reached the age of five.

And he warned that risked creating a new generation of violent criminals.

"We know who is going to be the next criminals. If we don't do something about it now that dysfunctionality is going to break all of our communities," he said.

"I personally believe family life has, at its heart, married life, and married life has been undermined by the present Government quite considerably both in tax and benefits."

Conservative leader David Cameron, who set up the social justice policy group to explore future policy options, said the Breakdown Britain report was "powerful and convincing".

He pledged to work to reinforce marriage and reduce divorce rates in a bid to tackle educational failure, drug abuse and debt problems.

But Labour ministers derided it as a "crazy" return to John Major's "back to basics" policies that hid an approach that would "undermine hard-working families".

Tony Blair's official spokesman said the Prime Minister recognised the existence of a "relatively small group of people who are falling through the net".

"That's precisely why we published a social exclusion plan which looked at intensive support for the poorest in society and why we appointed an independent adviser on child poverty," he said.

"The key in terms of the Government's response has been to target help on those who fall through the net.

"The Prime Minister's belief is that you have to bear the responsibility of bringing up children and therefore it is the best way in which to do that which is the issue.

"But equally, you have to provide targeted help for those who fall through the net."