Nearly half of unmarried couples who live together wrongly believe they share the same rights as others who have tied the knot, according to a survey.
Some 48% of people who cohabit mistakenly think they have a common law marriage, the British Social Attitudes Survey of people in England and Wales discovered.
Experts said the misconception increases the risk of "severe financial hardship for the more vulnerable party" after a breakup.
The survey, by the National Centre for Social Research, found 46% of the public as a whole believe couples who live together share a common law marriage - just 1% less than in 2005.
Cohabiting couples account for the fastest growing type of household in England and Wales, the researchers said.
Anne Barlow, a family law and policy professor at the University of Exeter, which commissioned the study, said simply living together grants "no general legal status" to couples.
She added: "The number of opposite-sex cohabiting couple families with dependent children has more than doubled in the last decade, yet whilst people's attitudes towards marriage and cohabitation have shifted, policy has failed to keep up with the times.
"The result is often severe financial hardship for the more vulnerable party in the event of separation, such as women who have interrupted their career to raise children."
Only 41% of the 2,105 people who took part in the survey, funded by the Economic and Social Research Council, knew there was no common law marriage between cohabiting couples.
More than half of households with children (55%) thought unmarried couples shared the same rights as those who were married, compared to 41% without.
Singles were most aware of the law, as only 39% believed a common law marriage existed.
More men (49%) than women (44%) thought unmarried couples who live together were in a common law marriage.
People with religious beliefs were also more likely to be mistaken than those without (49% vs 44%), as were those without a university degree (50% vs 39%).
Prof Barlow said: "It's absolutely crucial that we raise awareness of the difference between cohabitation, civil partnership and marriage and any differences in rights that come with each."
Meanwhile, 28% of 18 to 24-year-olds and 39% of people 65 and over perceived cohabiting couples to be in a common law marriage.
More than half (52%) of 25 to 64-year-olds thought married and unmarried couples shared the same rights.