Married couples would get a tax and benefits boost worth thousands of pounds per year under a Conservative strategy unveiled today to avert Britain's family and social breakdown, which the party claims costs the taxpayer billions.

A major policy review commissioned by David Cameron, which was headed by the former party leader Iain Duncan Smith, called for spouses to be allowed to transfer any unused tax allowances to their partners.

They said the move would particularly help single-earner couples, where one parent stayed at home with young children.

In a move likely to prove controversial with the Tory right, Mr Duncan Smith indicated that the plans would probably also apply to gay couples in civil partnerships. When questioned on the matter, the MP acknowledged that the unions, which provide gay couples with legally enshrined rights, are "here to stay", although the report does not mention the issue itself.

The proposals - set out in a bumper 671-page report entitled Breakthrough Britain - are designed to remove what the document describes as disincentives for couples to stay together.

It claims that the transferable personal tax allowance, which will be worth £20 per week to one-earner households, would provide symbolic recognition for the institution of marriage. "It would indicate that marriage is valued because of its benefits to children and wider society," it adds.

The plans also suggest increasing couples' allowances through working tax credits, which could give 1.8 million couples an additional £32 per week; a trebling of child benefit for the first three years for all parents; and getting more single parents off benefits at an earlier stage and back into work.

Special marriage guidance sessions before a union takes place would also be initiated, giving people advice on how to work through problems.

The report claims that the moves in support of marriage and the family would help to remove a large part of the cause for social breakdown within society, which the report says costs the UK economy £102bn a year. Family breakdown itself takes up £24bn, crime £60bn and educational under-achievement £18bn, it adds.

Mr Duncan Smith dismissed claims that his group was "finger wagging or moralising," adding that marriage as an institution provided crucial stability to the lives of children.

"The Government has jigged the system so that lone parents do reasonably well on the way into part-time work, but couples don't at all on benefits," he said.

"You get heavily penalised if you’re a couple living together if one of you wants to stay at home, perhaps in the first two or three years and look after your child or to look after an elderly relative. You get heavily taxed.

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