"If the North American churches don't accept the need for moratoria, then to say the least, we are no further forward," he said at the once-in-a-decade Lambeth conference in Canterbury.
"That means as a communion we continue to be in grave peril."
Williams also asked Anglican church leaders after concluding the 20-day assembly to refrain - for now - from adopting official prayers for blessing same-gender unions.
|Robinson became the first openly gay Anglican bishop in 2003 [EPA]
The Anglican Church, with 77 million members, has been split since 2003 when the US Episcopal Church consecrated the first openly gay bishop, Gene Robinson, of New Hampshire.
More than 200 theologically conservative bishops boycotted the Lambeth gathering in a row over the gay clergy issue.
Last month conservative Anglican leaders staged their own conference in Jerusalem where they pledged to form a council of bishops as an alternative to churches who they say are preaching a "false gospel" of sexual immorality.
The 650 bishops at Lambeth said in a statement that "there is widespread support across the communion" for an extended moratorium on gay bishops and on blessing ceremonies for same-gender couples.
Williams also faces another battle in the Church of England, the Anglican mother church, over plans to ordain women bishops that have sparked threats of a mass walkout by conservative clergy.
The Anglican communion is the third-largest religious group in the world, behind Roman Catholics and Orthodox Christians.
Many Protestant churches are also struggling with how they should interpret what scripture says about gay relationships and other issues.