An estimated one in six people are now cohabiting with their partner as the number of married couples in England and Wales continues to decline.
Data from the Office for National Statistics (ONS) shows husbands and wives now make up less than half of the population, with 51.8% of adults single, divorced or widowed.
The ONS report said: "One of the main reasons for the decrease in the married population and the increase in the single population is the growth of cohabitation by unmarried couples.
"In the early 1960s in Britain fewer than one in a hundred adults under 50 are estimated to have been cohabiting at any one time, compared with one in six in 2010."
The figures show that in 2010, 48.2% of the adult population of England and Wales were married (21.6m people), 35.6% were single (16m people), 9.3% were divorced (4.2m people) and 7% were widowed (3.1m people). The number of couples getting married has been steadily declining since the 1970s.
Anastasia de Waal, of the Civitas think-tank, said: "When interpreting these statistics it's crucially important to factor in the gap between what couples want to do and what they are doing in practice.
"Attitude surveys repeatedly show us that the majority of young people today would like to marry, an aspiration which is notably highest amongst cohabiting couples. In harder times, many couples feel that they are not able to afford the requisites - not so much the wedding itself, but things such as owning a home and a car.
"As such, steady rises in unemployment and the cost of living over recent years may well be a key contributor to a continuing drop in the number of married couples."
The ONS report said there is also "some evidence" of a fall in the number of marriages ending in divorce in recent years. Just under 114,000 couples were granted a divorce in 2009, the lowest figure since 1974.
People are most likely to get divorced aged 53, according to the figures, and there is a larger number of divorced women than men. The report said: "One possible reason for this is that more divorced men remarry each year than divorced women, leaving more divorced women in the population."