Cohabiting couples with and without children are the fastest-growing UK family type, official figures show.
Younger people are more likely to cohabit
The Office for National Statistics' (ONS) study found the number of cohabiting families increased by 65% in the 10 years up to 2006.
It also found that during the same period the number of married people with or without children fell by 4%.
The study said family types varied according to age, with young people more likely to cohabit with a partner.
The Focus on Families study found cohabiting families increased in the decade up to 2006 from 1.4 million to 2.3 million, while the number of married families fell to 12.1 million.
But despite the rise, married couples with or without children are still biggest family group - accounting for 71% of the UK's 17.1 million families.
Younger people were more likely to cohabit, the ONS said, with half of cohabiting families in 2001 headed by a person under the age of 35.
Some 21% of women aged 25 to 29 between 2001 and 2003 had lived with a partner before they were 25, the study found.
This compared with 1% three decades ago.
This increase in cohabitation did not fully explain the decline in marriage, the ONS said, because its research found fewer women aged between 25 and 29 were forming any union - either marriage or cohabitation - before the age of 25.
"This suggests a delay in partnership formation for younger generations of women," it said.
The two groups least likely to have got married before the age of 45 are men with no qualifications and highly-qualified women, the ONS said.
The report suggested such living arrangements could have an effect on health.
"Partnership continues to be the healthiest state in general. There are health benefits associated with partnership, especially marriage, but there are variations by sex," it said.
"In particular, older single women have better health then married women on many indicators of health status."
Meanwhile, the research found lone parents increased by 8% to 2.6 million, with lone mothers tending to be younger than lone fathers.
One in three lone mothers and one in ten lone fathers were under 35, the ONS said.
The study also found that by 2006, the average number of children in a family had fallen to 1.8 - down from 2.0 in 1971.
Married couples were found to have larger families, with 1.8 children, while cohabiting couples had an average of 1.6.