Religious leaders across Britain will hold a peace vigil as soon as war is declared.
Representatives from the Christian, Sikh, Muslim, Hindu, Jewish and Buddhist communities will lead prayers outside Birmingham Cathedral.
The city's bishop, the Right Reverend John Sentamu, called the leaders together and will be joined by the Roman Catholic Archbishop of Birmingham, the Right Reverend Vincent Nichols.
The religious seniors say they will meet in the square outside the cathedral at 4pm on the day war is announced. A cathedral spokesman said they would call for peace in what they still considered an immoral war, and pray for the safe-keeping of all those involved.
The vigil is part of the faith leaders' anti-war policy, which they made evident in a joint declaration on February 3 at the Birmingham Peace Gardens.
It said: "We do not believe that evidence presented to date suggests Iraq poses an immediate threat to international security.
"Without compelling new evidence to the contrary, we contend that military action is not morally justified."
The spokesman added: "In our prayers and thoughts are all the innocent people - men, women and
children - who will bear upon themselves the appalling sufferings and horrors of war.
"Our prayers are also for all peoples, military and civilian, who will be caught up in this unjust war."
The vigil is expected to last just under two hours to allow anti-war protesters to join the Stop the War Coalition, who will gather at 6pm in Centenary Square.