THE Conservatives are to launch a crusade for personal morality to try to halt what they say is a breakdown in traditional family values.

It comes in the wake of a Tory report that says unmarried parents are driving a generation of children into crime and drug dependency.

Dominic Grieve, the shadow attorney-general, said last night that people who tackle teenage yobs should not be prosecuted for assault. He added that strict Victorian values on family life had in some ways been successful.

The Tories claim the rise in cohabitation and single parenthood is unleashing a social and economic crisis.

In an appeal to grassroots supporters, the party will this week put the promotion of marriage back at the heart of its agenda, warning of dire consequences if more couples are not encouraged to wed.

A report commissioned by David Cameron, the Tory leader, claims the breakdown of the family is driving boys into the arms of street gangs at an annual cost to the country of more than £20 billion.

It seems to fly in the face of Cameron’s “hug a hoodie” message. But Tory strategists hope the findings will reassure long-standing Conservative voters that the new leader has not abandoned traditional party values.

The report is likely to be seized on by Labour as evidence that, below the surface, Tory social attitudes have not changed under Cameron and that the party continues to disapprove of many modern families.

It is the first heavyweight submission by the panels Cameron set up when he became leader to thrash out party policy. The interim report of the Social Justice Policy Group, headed by Iain Duncan Smith, gives an insight into the possible elements of the party’s next election manifesto.

It warns that family breakdown, drug and alcohol addiction, welfare dependency and educational failure have created an underclass mired in misery and “cut off from much of mainstream society”. The burgeoning underclass also “threatens the wellbeing of middle-class people living in once tranquil neighbourhoods”.

The report suggests that without a radical reappraisal of government policy towards marriage and the family, social tensions will grow, fuelling violent crime.

Duncan Smith said: “Critically, the report points out that at the heart of stable families and communities lies marriage. For too long the issue has been disparaged and ignored and its erosion has had a detrimental effect on us all.”

According to an academic study, nearly half of cohabiting parents split up before their child’s fifth birthday, compared with one in 12 married couples. The report says 70% of young offenders come from lone-parent families.

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