According to solicitor Muriel Walls, cohabiting couples fall into three main categories:

1. Young professional couples: These often drift into cohabiting. They have independent incomes, and have regularly spent nights in each other's house or apartment until such time as there were more clothes in one than the other. Some of these may live together for a couple of years before deciding to get married.

2. Battle-scarred divorcees or separated people: They have been through one painful and costly separation and do not want to enter into a new set of legal obligations. They might also not wish to re-open the scar of the old one by seeking a divorce in order to remarry, so they establish a new family on an informal basis.

3. Conscientious objectors: Some couples make a conscious decision not to marry, perhaps because they associate marriage with implied inequality between the spouses, or because they see their relationship as essentially a private matter.

All three categories of couples may have the illusion that such an informal situation is easier to walk away from, but this is not so, according to Walls.

"Some couples live together for a few years and then get married," she says. "But if they are together for five or six years their relationship will suffer the same stresses and strains as anyone else's. These relationships get into trouble, and the emotional trauma of their break-up is no less than that of a marriage."

By then their finances will have become intermingled; they are unlikely to have been keeping receipts for every item of domestic expenditure. One might have been paying the mortgage, for example, while the other paid all other household bills. So when it comes to proving the financial contribution each made to the relationship, there will be insufficient documentation.

A legal framework exists for dealing with the issues that arise when a married couple separates, but there is none for cohabiting couples.

Walls sees no immediate likelihood of such couples acquiring legal rights, and points out that this issue is only now being addressed in England. In the meantime, she would advise all such couples, at the very minimum, to sort out the ownership of the family home and the guardianship of any children, and to make a will.