Millions of unmarried couples who live together could be unaware of their rights if the relationship breaks down.
Resolution, a family law group, carried out a survey which found two-thirds of cohabiting couples wrongly believe "common-law marriage" laws exist when dividing up finances.
The number of unmarried couples living together has more than doubled from 1.5 million in 1996 to 3.3 million in 2017.
Resolution chairman Nigel Shepherd said current laws were "behind the times". He said: "The government must listen to the public, legal professionals and a growing number of politicians who all agree that we need reform to provide basic rights to cohabiting couples should they separate." Mr Shepherd said "society has changed", as cohabiting couples have become the fastest-growing family type in the UK.