Unmarried couples who live together could win rights to share each other's wealth if they split up under proposals being unveiled.
The Civil Partnership Act offers similar protections to gay couples
Rights to a share of property and pensions, to claim maintenance and lump sums could be among new measures.
The law reform body the Law Commission is to publish a consulation on the proposal with a final report in 2007.
About four million people cohabit but do not have the rights of people who are married or in civil partnerships.
Some fear such measures would undermine the status of marriage.
Cohabitees could make the same financial claims as divorcees but on a less generous basis, under the proposals being considered by the independent body.
The commission suggests that the rights should apply to those who have lived together for a certain period or who have a child.
Lawyers have suggested the entitlements should apply after couples have lived together for two years although this is being consulted on.
Cohabitees can currently claim maintenance for a child but not for themselves.
Entitlement to inheritance and pensions are also among the areas being examined.
The Civil Partnership Act, introduced last year, offers similar legal and financial protection to gay couples.
The number of people who are living together instead of getting married is expected to double over the next 15 years.
LIVING TOGETHER TRENDS
2.2 million cohabiting couples in 2005
The peak ages for cohabitation are 30-34 for men and 25-29 for women
The Office for National Statistics predicts there will be 2.93m cohabiting couples by 2021
That is a rise of almost 90% in 25 years
The 2001 Census showed the number of households which are home to married couples fell by 10% in 10 years, to 45%
Already more than three-quarters of couples live together before marriage, and one in four children is born to parents who are cohabiting.
Many couples who live together will welcome any moves to improve their rights.
Rose Green lived with her partner John for more than twelve years when he died unexpectedly.
She told BBC Radio 4's Today programme she was left in a "vulnerable position" because they had never married and her partner had not adjusted his will.
"I started to realise there was very little protection for me in the situation I was in.
"Had we been married I would have been his next of kin, I would have been able to claim his pension, and I would have inherited our home."
She said if they had known there was an easy way to register their commitment to each other, such as a Living Together Agreement, they would have opted for it.
Barbara Simpson, a deputy district judge in the family division and leading family law expert, said new rights would recognise there is little difference between living with a partner for years and looking after children - and doing the same as husband and wife.
"This will address a terrible unfairness, and it's long overdue," she said.
She said the Law Commission would want to differentiate between different types of co-habiting couples, such as short term versus long term.
But Daily Mail columnist Melanie Phillips told the BBC changing the law would undermine marriage.
She said: "This idea that society is changing and therefore the law has to change to keep up with it is wrong, in my view, historically the law has led the progressive dismemberment of marriage by stripping it progressively of meaning.
"And this is but the latest example of that. The law is based on justice; justice requires that you don't get something for nothing. You don't claim rights if you don't enter into obligations."