Rowan Williams advocates a period of "gracious restraint"
The Archbishop of Canterbury has expressed his concern at the election of the second openly gay bishop in the Anglican Church.
Rowan Williams said the move raised "very serious questions" for both the Episcopal Church in the US and the Anglican Communion as a whole.
But he noted that the election of the Reverend Mary Glasspool in the diocese of Los Angeles had yet to be confirmed.
The 2003 election of the first openly gay bishop created a massive rift.
Ms Glasspool was elected on Saturday as an assistant bishop in the diocese of Los Angeles.
She needs a majority of national Episcopal Church heads to back her consecration.
The Episcopal Church leader, Presiding Bishop Katharine Jefferts Schori, has said she will consecrate any bishop whose election follows the rules.
In a statement posted on his website Dr Williams said the election "raises very serious questions not just for the Episcopal Church and its place in the Anglican Communion, but for the Communion as a whole".
Robert Pigott, BBC religious affairs correspondent
Despite intense pressure to maintain a moratorium on ordaining gay bishops - the Episcopal Church's ruling convention voted in July to allow homosexual men or women to become bishops.
This clarification of the Church's policy makes it all the more likely that Mary Glasspool's election will be confirmed by its bishops and a committee representing the Church's lay and clergy members.
If it is confirmed, Canon Glasspool's election will set back attempts to preserve worldwide Anglicanism intact and makes it more probable that the American Church will be consigned to membership of the second of a two-tier Communion.
"The process of selection however is only part complete," he said.
"The election has to be confirmed, or could be rejected, by diocesan bishops and diocesan standing committees. That decision will have very important implications."
Bishops of the US Episcopal Church voted in July to overturn a three-year ban on the appointment of gay bishops.
Anglican leaders had asked the Church to observe the moratorium.
"The bishops of the Communion have collectively acknowledged that a period of gracious restraint in respect of actions which are contrary to the mind of the Communion is necessary if our bonds of mutual affection are to hold," said Dr Williams.
Ms Glasspool, 55, has been a canon in the Diocese of Maryland for eight years, according to a statement by her on the website of the Episcopal diocese of Los Angeles.
It says she has been with her partner, Becki Sander, since 1988.
Traditionalists have already expressed opposition to the latest election.
Conservatives insist the Bible unequivocally outlaws homosexuality, while liberals believe the Bible should be reinterpreted in the light of contemporary wisdom.
Conservatives were incensed by the election of the first openly gay bishop, Gene Robinson of New Hampshire, six years ago
The row led to the formation of a conservative breakaway Episcopal movement in the US - the Anglican Church in North America.
As head of the worldwide Anglican community, Dr Williams has been under pressure to recognise it.
The traditionalists have formed a range of new alliances, often with conservative churches in Africa.