Family comes first, says Cameron

This will mean not only tax breaks for married couples and changes to the benefit system to make it pay for parents to stay together, but also family-friendly working hours, affordable childcare, moves to promote couple counselling and an insistence that learning about relationships forms part of all sex education at school, he will say.

In a speech in London, Mr Cameron will make an explicit link between high rates of family breakdown and social problems like crime, anti-social behaviour, drug addiction and poverty.

And he will say that the family is "the best institution in our country" to counter these social ills, because of its vital role in bringing up children.

Mr Cameron's comments come as a report from the UK Children's Commissioners warns that Britain's children feel increasingly unsafe, drink more alcohol and face more pressure at school than elsewhere in Europe.

Speaking to the relationship guidance charity Relate, Mr Cameron will say: "The number one challenge we've got in this country today is to strengthen our society. There is no more important way of doing that than strengthening families, and there's nothing more important to families than the strength of their relationships.

"Today, we have the report from the UK's Children Commissioners showing that something terrible has gone on with our society these last few years. Right now, Britain has one of the highest rates of family breakdown in Europe. And we also have some of the worst social problems in Europe.

"That's why I say it's time for change: to make this country more family-friendly so we can turn around the social breakdown, turn around the crime and anti-social behaviour, turn around this unacceptable situation where our cost of living's going up and the quality of life is going down.

"I don't think we'll ever get to the heart of the big problems we face, from crime and anti-social behaviour to welfare dependency and educational failure; from debt and drug addiction to entrenched poverty and stalled social mobility, if we don't help the best institution in our country - the family - do the vital work that it does in bringing up children.

"What that help is - and how it is delivered - will be amongst the defining social reforms of the next Conservative Government."