Couples will be encouraged to marry rather than live together under Conservative proposals derided by opponents as a return to John Major's 'back to basics' approach.

David Cameron, the Conservative leader, has endorsed a 300-page report published today by former leader Iain Duncan Smith which claims that family breakdowns cause social problems costing more than £20 billion a year.

In the 1990s, a string of sex scandals blighted Mr Major's government after he launched a "back to basics" campaign that was interpreted as a moral crusade. The Labour Party claims that the Tories are returning to that ill-fated approach.

And campaigners for lone parents said the Tories risked stigmatising people who did not live in a traditional family set-up while one think-tank warned that it was naive to think that supporting marriage was the key to easing society's ills.

Mr Duncan Smith had been shaken to discover that nearly half of cohabiting couples had broken up by the time their first child was five, whereas the figure for married couples was much lower. This supported his belief that the Conservative Party must ensure their policies supported marriage and encouraged couples to stay together.

"If marriage rates went up, if divorce rates came down, if more couples stayed together for longer, would our society by better off? My answer is yes. And so I will set a simple test for each and every one of our policies: does it help families?"

Family breakdown - fuelled by Government policies such as tax credits - had led to the creation of a growing underclass in society and a violent crime wave that was tearing communities apart, according to Mr Duncan Smith.

"As this report shows, children from a broken home are twice as likely to have behavioural problems, perform worse at school, become sexually active at a younger age, suffer depression and turn to drugs, smoking and heavy drinking."

Launching the Breakdown Britain report in central London today, the former leader said some of the issues he had raised might cause colleagues in the party to "swallow hard", but they would also realise they needed addressing.

He said: "This is not about finger-wagging, or telling people they’ve done wrong. What I’m trying to say is that there is a better way.

"The modern society, the 21st century society we wish to live in, needs to be more cohesive and balanced than I believe it is at the moment."

Mr Cameron, who set up the social justice policy group to explore options, has already welcomed the Breakdown Britain report as "powerful and convincing".

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