Unmarried couples who split up are likely to win new rights to divorce-style demands for financial support and a share of their former partner's property.
The proposals from the Lord Chancellor Lord Falconer are planned for a draft Bill in two years' time and will go further than any previous legislation in providing legal protection for co-habiting partners.
The plans are expected to be criticised for undermining the institution of marriage but two Birmingham lawyers said the measures did not go far enough.
Victoria Jones, a family solicitor at Irwin Mitchell, said she believed the proposals would still not protect heterosexual couples if their relationships ended.
"The legal position is a throwback to a bygone age when a couple's union was not considered legitimate unless they were married," said 30-year-old Miss Jones.
"It still will be with the proposals outlined by the Lord Chancellor. The courts have no power to transfer capital assets between partners or order maintenance payments if an unmarried couple's relationship breaks down.
"It doesn't actually matter how long they have been together.
"You could live with a man for decades, change your name to his by deed poll, have his children, give up your job and you would still not have any financial claim against him if the relationship ends. The same lack of monetary rights is true for the man."
Her colleague, Amanda Holden, a 35-year-old conveyancing solicitor, said unmarried couples would still lose out on inheritance tax advantages.
Mrs Holden advised couples to own their homes on a joint tenancy, rather than expect the law to turn in their favour.
She said: "If a couple own as joint tenants and one of them dies, the surviving tenant would automatically acquire the whole of the property regardless of what is set out in the deceased person's will.
"If the property is sold, the proceeds are split equally regardless of the amount each party has contributed to its initial purchase and mortgage payments."
The four million, or one in six, adults who co-habit often only realised how few rights they had when it was too late, Miss Jones said.
"Worryingly many believe they have the same rights as husbands and wives, in fact the notion of a common law marriage was abolished in 1753."