Number of cohabiting couples soars by two thirds under Labour

It found that one in five women chooses to live with a man rather than marry or stay single while they are in their early 20s.

The rise of cohabitation has gone alongside a continuing decline in marriage, the report from the Office for National Statistics said.

The number of single-parent families has also leapt, by 8 per cent since 1996, to reach 2.6million, it added.

The analysis follows David Cameron's promises to reverse the decline of family life. This is likely to be a key election issue if Gordon Brown opts for an autumn poll.

The Tory leader has said he will bring in tax breaks for married couples and end the bias in favour of single mothers in the benefit system in order to restore the role of the family as "the best welfare system".

His plans are based on overwhelming evidence that children brought up by two parents are more likely to enjoy good health, do well at school and get qualifications and good jobs.

The ONS analysis said that in the ten years between 1996 and 2006 - a decade dominated by Tony Blair's government which came to power in May 1997 - the number of cohabiting couples rose by 65 per cent from 1.4million to 2.3million.

It acknowledged: "It has been found that there is an increased risk of union breakdown among cohabiting couples compared with married couples."

The number of married couples fell by 4 per cent to 12.1million. Over the same period, there were 8 per cent more lone-parent families, with numbers rising to 2.6million.

The spread of unmarried relationships means there are around half as many cohabiting couples or families and single-parent families put together as there are married couples.

The decline in marriage is not entirely caused by the rise of cohabitation, the analysts said.

Fewer women are now marrying or living with men before 25, which "suggests a delay in partnership formation for generations of young women'"

The increase in single young women is linked to their greater educational achievements and ever-expanding career opportunities.

However, while women are putting off marriage, more than one in five have lived with a man before the age of 25. In the 1970s, only one in 100 women did so.

An average cohabitation is thought to last around three years, while a typical marriage runs for 11 years.

The ONS said married couples bringing up their children had the greatest number staying on at school after 16 - seven out of ten boys and eight out of ten girls.

The report added: "There are benefits in partnership, particularly marriage."

Mr Brown's tax credit benefits system, introduced from 1998, has been blamed for persuading many mothers to stay single because it pays more to lone parents than to couple families.

Labour has stripped away the last tax break for the married, the Married Couples Allowance, and tried to downgrade the importance of marriage, removing references to it from official documents, and making register offices replace signs mentioning weddings with ones alluding to "ceremonies".

• There is no tax or benefit incentive for couples to marry as opposed to living together as cohabitees.

Many cohabitations are thought to break up because the benefit system is heavily skewed in favour of single people.

Mr Brown's tax credit system pays single mothers who work more than 16 hours a week and provides them with generous childcare allowances.

But tax credits make no provision for a second adult in the house, and the income of a working second adult means lower tax credits.

Labour MP and former welfare reform minister Frank Field has calculated that a single mother with two children under 11 on the minimum wage received tax credits last year that took her weekly income to £487 if she worked only 16 hours a week.

A two-parent family with one earner would have to put in 116 hours of work on the same pay to get the same money.