More people are living alone, more children are being raised by single parents and more grown-up children are living with their parents than ever before, according to the Office for National Statistics.

One expert said that the in-depth annual study was final confirmation that the nuclear family had become “a museum piece”.

The Social Trends report made clear, however, that the most radical changes had been to child-rearing and marriage.

Its figures showed that 30 per cent of women under 30 had given birth by the age of 25, while 24 per cent had married: the first time that having children had become the first major milestone of adult life, ahead of marriage. This was in sharp contrast to their parents’ generation. In 1971 three-quarters of women were married by 25, and half had given birth. The statistics also showed that:

Š the number of adults living alone doubled in a generation, from 6 per cent to 12 per cent, because of a combination of death, divorce and marrying at a later age;

Š single-parent households nearly tripled from 4 per cent of the total to 11 per cent between 1971 and 2008;

Š the percentage of households comprising the traditional nuclear family – a couple with children – fell from 52 per cent to 36 per cent over the same period;

Š the number of married couples hit the lowest level, in real terms, since 1895, with 237,000 marriages in England and Wales in 2006, down from a peak of 471,000 during the Second World War;

Š some 1.66 million children were being brought up by an unmarried couple, up from one million 10 years ago. The number brought up by married parents dropped from 9.57 million to 8.32 million.

The figures were published two months after official statistics showed that the annual rate of teenage pregnancy in England had risen to 42 in every 1,000, despite a £286 million government campaign to tackle the problem. The figures reinforced Britain’s position as the teenage pregnancy capital of Europe.

Dr Richard Woolfson, a leading family expert and child psychologist, said: “The nature of family life has changed significantly in the last 30 years. The traditional nuclear family of two parents and 2.4 children has become a museum piece.

“The single-parent family carried all sorts of social and moral judgments back then [in 1971]. That is just not the case any more.

“The couple who do not get married is now socially acceptable in a way that it never was before.”

Dr Woolfson said it was impossible to say if children raised outside the traditional family were unhappier, but he added: “You have to ask what sort of families will today’s children create. Where will they go?”

Iain Duncan Smith, the former Conservative leader and chairman of the Centre for Social Justice, said the figures “mattered hugely”. He added: “One in 12 children will experience their parents breaking up by the age of five if those parents are married. But that figure is one in two if the parents are cohabiting. Marriage is not just a piece of paper.”

Mr Duncan Smith called for the tax system to favour those who choose marriage over living together. “It is not our job, as politicians, to lecture, but the problem has been caused by successive UK governments centring on the child, and forgetting the parent,” he said.

“We now know that children suffer hugely if they don’t get the balance of two parents in their upbringing. Those with two parents are less likely to take drugs, more likely to do well at school, more likely to get jobs.”

The ONS figures also showed that a rise in the number of people going to university and a decade-long house price bubble had meant that 300,000 more people under the age of 34 lived at home than in 2001. In 2008, 29 per cent of men under the age of 34 lived at home with their parents, up from 27 per cent seven years earlier.

The ONS said the Divorce Reform Act of 1969, which made it easier to dissolve a marriage, was one of the main causes for the radical change in families.

Critics of the Government point out that the tax system has also been altered to the advantage of unmarried couples.

The last tax break for married couples, the married couples’ allowance, was abolished in 2000. State benefits, and especially Gordon Brown’s flagship tax credit system, pay more to single mothers than to two-parent families.

Patricia Morgan, the author of The War between the State and the Family, said: “There is discrimination on the one hand, but on the other there are major benefit incentives for a single woman to have children. It’s a mug’s game, getting married.”

However, the ONS figures suggested that those who got married were staying together for longer. An average marriage lasted 11.6 years in 2005, up from less than 10 years in the mid-1990s.

Sue Palmer, a child expert and author of Toxic Childhood, said many of the statistics relating to the breakdown in the traditional family were linked indirectly to separate figures showing that 30 per cent of girls and 31 per cent of boys were overweight or obese. She said: “The more parents work, the more the children stay at home and are not playing outside with their friends.”

A spokesperson for the Department for Children, Schools and Families said: “We know that the make-up of the family unit is changing but that doesn’t mean the family is breaking down. It is Government’s role to support families in all shapes and sizes, which is why our policies are aimed at empowering and advising parents to make the best choices for their children. We are investing £250 million in local services for parents, particularly those in challenging circumstances and last December the first ever Government relationship summit looked at what more we can do to support children and families and give extra help to families experiencing relationship breakdown.”

A Department for Work and Pensions spokesperson said: "We support all struggling families, giving them the real help they need. Modern families come in all shapes and sizes but every family with one child, where one adult is working full-time at minimum wage took home £292 a week in 2008 compared to £182 in 1999 – a real terms increase of 24 per cent.

"Child benefit, Tax credits and other benefits help all working familiies and through the welfare reform bill we are putting new expectations on workless parents to ensure they take up the support we offer."