The Presbyterian Church (U.S.A.) has voted to allow its pastors to perform same-sex marriages and has approved the definition of marriage as between two persons as well as between a man and a woman.

The largest Presbyterian denomination in the United voted Thursday to change its definition of marriage allowing its clergy to solemnize same-sex ceremonies in states where gay marriage is legal.

The vote was passed by 429 to 175 votes by the delegates from the 1.76 million-member Presbyterian Church (USA). It makes the church the first big American denomination to unequivocally allow clergy to officiate at gay marriage ceremonies and to define marriage as between two persons.

The vote came during the denomination's once every two years General Assembly and it changes the denomination's Book of Order to describe marriage as being between "two people."

Conservatives are concerned the move could divide the church and lead to further membership declines, the Detroit Free Press newspaper reported.

It said that since 1992, Presbyterian Church USA has lost more than a million members, declining 37 percent from 2.78 million to 1.76 million last year. Congregations opposed to same-marriage have broken away, and more could leave after Thursday's vote.

The action effectively removes the ban on Presbyterian pastors marrying same-gender couples in jurisdictions where gay-marriage is legal, said the PC USA news service.

The text of the Book of Order says, "Exercising such discretion and freedom of conscience under the prayerful guidance of Scripture, teaching elders may conduct a marriage service for any such couple in the place where the community gathers for worship, so long as it is approved by the session; or in such other place as may be suitable for a service of Christian worship.

"In no case shall any teaching elder's conscience be bound to conduct any marriage service for any couple except by his or her understanding of the Word, and the leading of the Holy Spirit."

The Presbyterian Church is one of the largest Protestant denominations in the United States, but trails far behind the Southern Baptist Convention in size with around 16 million members.

However, eight U.S. presidents, including Ronald Reagan and Dwight D. Eisenhower, have been Presbyterians.

The Stated Clerk Gradye Parsons, who is the effective chief executive of PC USA, said, "The conversations about sexuality began in 1978." In that years, he said, there was a church declaration that "homosexuality does not accord with God's plan for humanity."

Parson's noted, "There have been places along the way when our talk turned to action, and this is one of those days."

He added, "Both the church and the society have changed – more people are getting to know gays and lesbians, laws are changing and pastoral situations are changing."

Presbyterians from Africa and the Arab countries said that many people where they live do not approve of homosexuality, and this concerned them.