Brighton Kemp Town MP Des Turner is backing a Westminster rebellion against war on Iraq.
The Labour MP has signed an amendment that states Prime Minister Tony Blair has not yet proved the case for military action.
The move puts him on course to vote against the Government in a vital House of Commons vote tomorrow night.
Last night Britain tabled a new draft resolution to the United Nations but the PM now wants Parliament to back a motion that warns Saddam he has his "final opportunity" to get rid of his weapons of mass destruction.
The motion says: "This House supports the Government's continued efforts in the UN to disarm Iraq of its weapons of mass destruction and calls on Iraq to recognise this as its final opportunity to comply with its disarmament obligations."
But Dr Turner, who was one of 53 MPs who rebelled on the issue in September, has signed an amendment to the motion.
It finds "the case for military action against Iraq as yet unproven" and is backed by about 80 MPs.
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