The Archbishop of Canterbury has warned that proposals by the Law Commission to give legal rights to cohabiting couples could create a rival to marriage.
Marriage has suffered a long process of erosion, said Dr Williams
He said the plans could have a harmful impact on family stability and the emotional development of children.
Dr Williams told the Sunday Times the government was the latest to fail to strengthen the institution of marriage.
He said marriage had "suffered a long process of erosion" and the plans added to confusion about its status.
"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."
Dr Williams acknowledged the "very proper concern" to provide for vulnerable "left stranded at the end of a partnership".
But he said people "anxious" about the needs of a partner could already make the decision to include them in a will or legal contract.
Under the Law Commission proposals unveiled last month, unmarried couples who live together could now win rights to share each other's wealth if they split up.
Rights to a share of property and pensions, to claim maintenance and lump sums are among measures suggested in the consultation paper by the commission, set up by the government to recommend legal reform.
BBC religious affairs correspondent Robert Piggott said the remarks were a reflection of Dr Williams' alarm about the decline of marriage, the role of fathers in society, and the impact on children from poorer families.