The cleric widely tipped to become the next Archbishop of Canterbury has called any attack on Iraq "immoral".
Dr Rowan Williams, Archbishop of Wales, has signed a declaration criticising the West's war on terror and said an assault on Iraq would be "illegal".
The declaration was published in the Roman Catholic weekly paper, The
In it, Dr Williams and other signatories, including a Church of England and Catholic bishop, say attacking Iraq would be tantamount to fighting "terror with terror".
"It's deplorable that the world's most powerful nations continue to regard war, and the threat of war, as an acceptable instrument of foreign policy," they write.
They add that such thinking is in "violation of both the United Nations and Christian moral teaching".
They raise their concerns amid increasing indications from the US that such an invasion is being considered.
Dr Williams has been reported as being first choice to replace Dr George Carey in the Church of England's top job.
He is believed to be favored by Prime Minister Tony Blair, with whom the final decision rests.
In January, the outspoken cleric described the Afghanistan conflict as "morally tainted" and "embarrassing".
He is a member of the Christian pressure group Pax Christi, which was behind the anti-war declaration.
The document will be presented to Downing Street next month.
Other signatories include the Church of England Bishop of Chelmsford, Reverend John Perry, and the Catholic Bishop of Brentford, Reverend Thomas McMahon.
They want Iraq to allow UN weapons inspectors to check if it had weapons of mass destruction.
They also want Britain and the US to do the same.
The signatories add: "It is our considered view that an attack on Iraq would be both immoral and illegal and that eradicating the dangers posed by malevolent dictators and terrorists can be achieved only by tackling the root causes of the disputes."