The National Association of Evangelicals (NAE), which represents about 45,000 churches across America, endorsed a declaration against torture drafted by 17 evangelical scholars. The authors, who call themselves Evangelicals for Human Rights and campaign for "zero tolerance" on torture, say that the US administration has crossed "boundaries of what is legally and morally permissible" in the treatment of detainees.

"Tragically, documented cases of torture and inhumane and cruel behaviour have occurred at various sites in the war on terror, and current law opens procedural loopholes for more to continue," the NAE said last night.

The strong alliance between Christian evangelicals and Mr Bush, an important key to his electoral successes, has been tested in recent months with the Mark Foley scandal over his attraction towards male teenage pages in Congress, and perceived corruption in parts of the Republican party. One in three white evangelicals voted for Democratic candidates in last November's mid-term elections, a rise on the 2004 presidential elections.

But Christian criticism has tended to come from the right of the Bush administration. Yesterday's statement on torture suggested a new determination on the part of the evangelical churches to detach themselves from the Republican party and stake their independence - even if it is from a position more traditionally associated with the left.

A leading figure within the NAE, Rev Richard Cizik, told the Associated Press that the statement condemning torture was not intended as a criticism of the Bush administration. But he added: "There is a perception out there in the Middle East that we're willing to accept any action in order to fight this war against terrorism. We are the conservatives — let there be no mistake on that — who wholeheartedly support the war against terror, but that does not mean by any means necessary."

The document on human rights quotes the Bible, Pope John Paul II and Elie Wiesel among other authorities. It particularly condemns the indefinite detention of suspects without trial.

"As American Christians, we are above all motivated by a desire that our nation's actions would be consistent with foundational Christian moral norms," the document says. "We believe that a scrupulous commitment to human rights, among which is the right not to be tortured, is one of these Christian moral convictions."