Leaders of the two main Christian churches have joined forces to confront Tony Blair over his claims that war on Iraq would be morally justified.
Archbishop of Canterbury Rowan Williams and Catholic Archbishop of Westminster Cormac Murphy-O'Connor have issued a joint statement expressing doubts about the "moral legitimacy" of the war.
They have also called for a continuation of weapons inspections.
The churchmen's intervention is a setback to Mr Blair, who responded to last week's million-strong march in London by vigorously asserting the "moral case for removing Saddam".
The archbishops' joint statement, drawn up at a recent private meeting, expresses "deep disquiet" about the possible consequences of an attack, which it said could lead to "unpredictable humanitarian and political consequences".
They said: "War is always a deeply disturbing prospect. One that can never be contemplated without a sense of failure and regret that other means have not prevailed, and deep disquiet about all that may come in its train.
"The events of recent days show that doubts still persist about the moral legitimacy, as well as the unpredictable humanitarian and political consequences, of a war with Iraq.
"We recognise that the moral alternative to military action cannot be inaction, passivity, appeasement or indifference.
It is vital therefore that all sides in this crisis engage, through the United Nations - fully and urgently - in a process, including continued weapons inspections, that could and should render the trauma and tragedy of war unnecessary."
Mr Blair is likely to hear a similar message from the Pope on Saturday, during a planned audience during his visit to Rome. John Paul II has publicly expressed grave concern about the prospect of conflict in Iraq.
PA