The Office for National Statistics (ONS) data suggests the number of people who cohabit has doubled to 2.9 million since 1996.
This includes both heterosexual partners and same-sex couples who have not had a civil partnership ceremony.
There are 12.2 million married couples, down 457,000 over the same period.
The statistics were contained in the Labour Force Survey - a study of households in the UK - which encompassed 102,421 individuals in 43,642 homes between April and June 2012.
The number of people aged 35 to 44 who cohabit increased from 7% to 15%.
"This may be related to the increasing age at marriage," the report stated.
Its findings included:
Some 38% of married couples have dependent children, and 39% of cohabiting different-sex couples have dependent children.
The report notes: "Although married couples are more likely to have children than cohabiting couples, they tend to be older on average than their cohabiting counterparts so children may be older and have left home.
"Cohabiting couples are more likely to be in the right age groups for childbearing.
"These two competing factors of age and likelihood of childbearing mean that a very similar percentage of married couples and opposite-sex cohabiting couples have dependent children."
There are now two million lone parents in the UK, up from 1.6 million, the survey suggests.
Some 29% of households are made up of only one person, while almost 20% are families of four or more people.
Couples living together without being married is the fastest-growing type of family in the UK, official figures show.