Too many women are relying on their status as "common law wife" to enable them to claim a share of their home and on-going support following a break down of their relationship.
The concept of common law spouses was abolished in 1753 , according to Meredith Thompson, a solicitor in the family and matrimonial team at the Birmingham office of Mills & Reeve.
The term has become part of the common vernacular, used by clerics, MPs and the Press, but in fact is meaningless.
She said: "The term is frequently bandied about but is a myth that has persisted for 250 years.
"I have had women come to see me who have spent two years with a partner and are convinced that this affords them the status of common law wife. It makes no difference if they have been there for two weeks or 50 years - there is no such legal status and no rights accruing to it."
If there are children, then the father will have a responsibility to ensure they have a roof over their head and to pay maintenance until the younsters cease full time education. He does not, however, have any responsibility to support their mother.
Ms Thompson said a cohabitation contract had to fulfil certain criteria before a court would enforce it.
"Principally it must comply with the rules of contract law, or be made by deed. The agreement must be fair and be restricted to financial and property matters, furthermore both parties must give each other financial disclosure and take independent legal advice," she added.
However, a partner was unlikely to want to tie himself down to long term maintenance and support payments.
Ms Thompson warned: "Women who are supported by a long term partner need to realise how vulnerable they are."