Senior Roman Catholics and evangelicals in the US have joined forces to warn of a grave "threat" to society caused by same-sex marriage.

In a statement to be published in the March edition of First Things, the US journal of religion, they describe the legalisation of same-sex marriage as "a graver threat" to society than divorce or cohabitation.

"We must say, as clearly as possible, that same-sex unions, even when sanctioned by the state, are not marriages," the statement, titled "The Two Shall Become One Flesh: Reclaiming Marriage," says. "Christians who wish to remain faithful to the Scriptures and Christian tradition cannot embrace this falsification of reality, irrespective of its status in law."

Influential Christians such as Rick Warren, pastor of Saddleback Church, Daniel Akin, president of Southeastern Baptist seminary and Timothy George, Dean of Beeson divinity school, are among the 30 senior Christian leaders who have endorsed the statement, according to Baptist Press.

The statement is the work of a consortium of Catholics and Protestants formed under the banner Evangelicals and Catholics Together, set up by the Institute on Religion and Public Life. The Catholics include George Weigel, senior fellow at the Ethics and Public Policy Centre.

Timothy George said the statement, headed: "The Two Shall Become One Flesh", represents three years' work by the group.

He told Baptist Press that Catholics and evangelicals "continue to disagree on lots and lots of issues. And the people who drafted this statement are well aware of those differences. We have not smudged them or pushed them under the rug."

The members of the consortium "felt the complementarity of man and woman in marriage under God" was an issue on which evangelicals and Catholics could present a united witness. "We felt we had to speak out on this issue," he said.

The statement sets out Bible teaching and concludes: "Marriage is a unique and privileged sign of the union of Christ with his people and of God with his Creation - and it can only serve as that sign when a man and a woman are solemnly joined together in a permanent union."

It says marriage is in crisis throughout the Western world.

"The revolution in our marriage and family law, already well advanced, marches under the banners of freedom and equality. But these noble ideals are here gravely misapplied. When society systematically denies the difference between male and female in law and custom, our fundamental dignity is diminished, the image of God within us is obscured, unreality becomes legally established, and those who refuse to conform are regarded as irrational bigots," the statement says.

It says gay marriage "threatens the common good" and "distorts the Gospel".

"Keeping in mind the obligation to speak the truth in love, we must find ways to distinguish true marriage from its distortion, and we must do so without abandoning the public square," the statement says. "We owe our fellow citizens a socially engaged witness to the truth about marriage, which, with the family, is the unalterable foundation of a healthy, humane society."