Christians have reacted with concern to calls from the House of Commons Women and Equalities Committee to strengthen the legal rights of cohabiting partners.
The recommendations were published last week following a public consultation and include proposals to reform current regulations so that cohabiting couples can inherit from a deceased partner.
The committee claims that the changes would especially offer greater protection to ethnic minority women who have had a religious-only wedding.
The report recommends that ministers review the inheritance tax regime so that it is the same for cohabiting partners as for married couples and civil partners.
If implemented, the changes would bring cohabiting rights more closely into line with marriage regulations, which give surviving spouses an automatic inheritance.
"Cohabitation is the fastest growing family type in England and Wales. In 2021 there were around 3.6 million cohabiting couples in the UK compared with 1.5 million in 1996," the report says.
"Whereas married couples and civil partners in England and Wales have certain legal rights and responsibilities upon divorce or death, cohabitants receive inferior protections.
"Notwithstanding the legal reality, many people believe in the so-called 'common law marriage myth', which is the erroneous belief that after a certain amount of time of living together, the law treats cohabitants as if they were married.
"On family breakdown, cohabitants must rely on complex property law and trusts principles. Schedule 1 to the Children Act 1989 is outdated, mostly benefits the children of wealthy parents and is in need of reform. On death, cohabitants do not automatically inherit from their partner."
The report adds, "It is time the law adapted to the social reality of modern relationships while still recognising the social and religious status of marriage."
Simon Calvert, deputy director at The Christian Institute, said it was wrong to place cohabitation and marriage on an equal footing.
"To treat cohabitees as if they are married is to undermine the institution of marriage, with its unique promises and its unique benefits to society," he said.
"'Living together' and being married are not the same. The public commitment made in marriage vows is the foundation for stable families and society. Marriage has consistently been shown to be best for adults and their children alike.
"Studies have shown that less than one in five of all cohabitations last more than five years, and only around one in 20 last over ten years – the rest either married or separated.
"One result of this instability is the sad fact that more than half of the children of cohabiting parents will experience their parents' separation by the age of five, compared to only 15 per cent for children of married parents."
The committee received nearly 380 submissions from campaigners, academics and members of the public.
In its submission to the committee, The Christian Institute said that the attempt to equate the legal rights and responsibilities of marriage with cohabition "demonstrates a low view of marriage".
"It is incumbent upon government to ensure children are given the best opportunity by supporting, protecting and prioritising marriage among parents. Research consistently shows that children do best when their mum and dad are married," it said.