Children and society in general do better when parents are married, a new report from the Centre for Social Justice (CSJ) says.
The 'Family Structure Still Matters' report says that even when controlling for income and education, children who grow up in "unstable" families "suffer worse health" and are more likely to be excluded, join a gang and "end up as NEET" (Not in Education, Employment or Training).
According to the CSJ, cohabiting households are the "fastest growing family type" in the UK.
Despite this, married parents are twice as likely to stay together as cohabiting ones, the CSJ says, with over half (53%) of children in cohabiting households seeing their parents split by the age of five, compared to just 15% of children in married families.
"These differences matter because family stability has been shown to profoundly affect children's outcomes," said Cristina Odone, head of family policy unit at the CSJ.
She warns that the cost to the NHS, criminal justice system and treasury in terms of lost revenues is "huge" and that the anti-social behaviour of young people from broken homes is having an "equally corrosive" impact on society.
She calls on the Government to acknowledge the distinctive benefits of marriage to children and society.
"The consequences of family instability are alarming; while the benefits conferred by marriage are inspiring. It is therefore surprising that government consistently fails to distinguish between marriage and cohabitation," she writes.
"In its language around family structure, including, crucially in its data collection, government persists in blurring the two categories of 'married' and 'cohabiting'.
"Official silence on this issue has sent out the message that marriage and cohabitation are interchangeable. Yet we have seen how the two structures lead to widely different outcomes.
"By ignoring this distinction, the government risks robbing couples of making an informed choice about what kind of relationship they should embark on."
In its analysis, the report says that married couples enjoy healthier lifestyles and higher earnings, as well as greater levels of relationship satisfaction.
They are also more likely to volunteer, help their neighbours and "be more engaged" in their local community.
"The differences between cohabitation and marriage are not negligible. The government should stop pretending they are," the report said.