Couples who are married are less likely to split up than those who live together, the Office for National Statistics has revealed.
Analysis of Census results found that four-fifths of spouses who were married in 1991 were still together a decade later, compared with three-fifths of cohabiting couples.
The independent statistics body said older couples and those with children were more likely to stay in the same relationships.
The findings of the ONS are likely to reignite the political debate over whether married couples should be given tax breaks.
The Conservatives have pledged to recognise marriage in the tax system, ensuring that spouses would not lose out if one wanted to stay at home to raise their children, on the grounds that stable families are good for society.
But Labour ministers, who abolished tax breaks for married couples, say that families now come in "all shapes and sizes" and so it would be wrong to disadvantage single parents or widows.
The ONS report, included in a new edition of its regular publication, Population Trends, analysed the information about living arrangements provided by 318,533 adults in its own Longitudinal Study.
Researchers tracked those who answered questions in both the 1991 and 2001 Census then looked in detail to find out whether they remained in the same relationship over the decade.
They found that of those who were cohabiting in 1991, 61 percent were with the same partner 10 years later while 39 percent were not.
Of those who stayed together, most had by then married their partner. The majority whose relationship had ended were living alone.
By contrast, 82 percent of those who were married in 1991 were still with their spouse in 2001. Of the 18 percent who had divorced or been widowed, most were single although a small proportion had remarried.
"It shows that marriage was more stable than cohabitation, even when controlling for a variety of factors," the Telegraph quoted the ONS as saying.
Detailed models suggested that couples were more likely to split up if they were younger, had no children, had previously split up with partners, were poorly educated or were unemployed.
"The Government persists in saying there's no difference between marriage and other forms of relationship but these official statistics are telling us, from a completely neutral standpoint, that there is a difference," Jill Kirby, Director of the Centre for Policy Studies, a think-tank, said.
"Living together is not an equivalent to marriage for family stability. That's why it's important to protect and support marriage, in the interests of children," she stated.
Marriage rates are already at their lowest level since records began in 1862, with just 232,990 weddings taking place in England and Wales during 2008.
But despite the greater instability of cohabitation, the ONS believes even fewer couples are likely to tie the knot in future.
"There have been notable changes in UK partnership behaviour over the last 40 years," the report said.
"Divorce rates rose considerably during the 1970s, remained broadly stable after the mid-1980s, and more recently have fallen since 2004.
"At the same time, there has been a long-term fall in marriage rates since the beginning of the 1970s, and a steady increase in the proportion of adults cohabiting.
"For unmarried men in Great Britain aged 16 to 59, the proportion cohabiting increased from 11 per cent in 1986 to 27 per cent in 2007.
"There was a similar change for equivalent unmarried women, from 13 per cent to 28 per cent.
"This change in partnership behaviour is likely to persist.
"According to demographic projections, the long-term rise in cohabitation will continue, with the number of cohabiting couples in England and Wales projected to rise from 2.25 million in 2007 to 3.70 million in 2031.
"The same figures show that the proportion of the adult population that is legally married is projected to fall from 49 per cent in 2007 to 41 per cent by 2031," it concluded.