The Law Commission will announce a study of how to improve the rights of the growing number of people living together without marrying, which has now reached two million couples and is expected to increase to 3.8 million by 2031.

Previous attempts to review the law have been opposed by Tory backbenchers and church groups, limiting reforms solely to property rights.

The review, commissioned by the Lord Chancellor, is likely to face criticism that greater rights for cohabitees will undermine marriage.

Church of England traditionalists are expected to fight the plans. A church insider agreed that there would be opposition and a spokesman said: “The Church believes that marriage has a unique role and is important for society and the raising of children. We would not wish to see this diluted, but strengthened by government policy.”

But the General Synod has recognised that cohabiting couples need to be protected, and the Rev Simon Stanley, vicar of St Chad’s, York, who proposed the motion, said that it was not just unmarried heterosexual couples who needed protection. Others, such as sisters or carers and the person whom they are caring for, also needed legal rights.

Dominic Grieve, the Shadow Attorney-General, said that the Conservatives accepted that the law needed to be reviewed but would oppose a move to give cohabitants equal rights to married couples.

But ministers are concerned about the disadadvantages suffered by unmarried couples.

Many people wrongly believe that if they live together they eventually acquire legal rights akin to those of married couples — the myth of “common law” marriage.

Research in 2000 found that 59 per cent of cohabitees believed that common law wives and husbands had rights similar to married couples.

But an unmarried partner has no right to claim financial support from the other partner, no matter how long they have lived together, and no automatic right to a share of property held in the other’s name.

Cohabitees cannot inherit their partner’s assets free of inheritance tax, they do not always have parental rights, they are not automatically entitled to a share of assets after a split or to their partner’s pension after death. They do not count as legal next of kin.

Mark Harper, a leading family lawyer, said that the review would be welcome. “This is a golden opportunity for the Law Commission to bring the law in line with society’s attitudes.”

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