According to research commissioned by relationships charity OnePlusOne, 47 per cent of men and women, aged 18-34, think cohabiting couples have the same legal rights as their married counterparts.
Fifty eight per cent of people across all age groups do not know that 'common law' marriage is not a recognised legal status.
Penny Mansfield, director of OnePlusOne, said: "It's really concerning that when a lot of young people are thinking about settling down they're not fully aware of the legal status of their relationships.
"We often don't check out our rights because it seems unromantic, or even untrusting, to raise legal and financial issues about a relationship. But exploring the common 'what-ifs,' before difficulties arise puts couples and families on a more secure footing."
OnePlusOne has now launched a website, Married or Not, which is an online resource to outline the legal differences between marriage and living together.
It explains that cohabiting couples in the UK have virtually no legal rights in the event of death or relationship breakdown, though many are unaware of their lack of legal protection until they find themselves in desperate circumstances.
It also offers guidance for couples on bringing up difficult conversations regarding relationship status; helping them to strengthen their relationship by making plans for the future and discussing the 'what-ifs' – how they will ensure both partners are protected should the worst happen.
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