The Archbishop of Canterbury has warned that plans to give legal rights to non-married couples could further erode the institution of marriage.
Rowan Williams said the proposals by the Law Commission could create a rival to marriage and lead to the breakdown of family stability.
He told the Sunday Times that marriage had "suffered a long process of erosion" and the government was not doing enough to strengthen the important tradition of matrimony.
"The concept of cohabitation is an utterly vague one that covers a huge variety of arrangements," Dr Williams said.
"As soon as you define anything, you are creating a kind of status that is potentially a competition with marriage or a reinvention of marriage.
"I think one of the problems is trying to solve individual and infinitely varied problems by legislation."
Under new proposals revealed by the Law Commission last month, cohabiting couples would have the opportunity to share each other's earnings if they split up.
High court judge Roger Toulson, who is leader of the commission, said the plans would not damage the tradition of marriage.
He argued that they would, in fact, encourage people to wed because the financial responsibilities would be the same anyway.
Dr Williams said that the proposals showed "very proper concern for vulnerable people who are left stranded at the end of a partnership breaking up".
But he added that if people were "anxious" about the financial needs of their partner, they would probably have already drawn up a will themselves.