A campaign to eliminate the phrase "common law marriage" from official English was launched today.
A legal advice group warned the phrase has no legal standing anywhere in the UK yet is regularly misused in official forms.
It also features in a host of popular dictionaries, even though common law marriage is an "urban myth", said the Living Together organisation.
A survey for Living Together of 1,000 people showed 67 per cent mistakenly believed that cohabiting couples who lived together for a period became "common law husband and wife".
But disastrous situations can arise from assuming that co-habitation gives each partner the same legal rights as marriage, the group said.
More than four million people co-habit in Britain today.
Living Together spokeswoman Mary Webber said: "It's time for use of the term to be cut out completely.
"Everyday usage of the term perpetuates the myth which is damaging to many couples in the UK, because they believe they have rights that they don't have.
"There are plenty of steps cohabiting couples can take to protect themselves and each other, but people fail to realise that they are necessary."
For example, Rosemary Green, aged 59, had been living with partner John for more than 12 years when he died suddenly.
His family disagreed with Rosemary inheriting completely the home she had shared with him, leading to legal negotiations which lasted more than four years.