The motion submitted to the lower house of Parliament accused Koizumi of “employing absurd sophistry” to defend the move to send the troops to Iraq.
"We strongly demand an immediate resignation of the Koizumi cabinet," the motion said.
The Japanese “Self-Defence Forces (SDFs) will be dispatched primarily to support US and British troops. It will be inevitable that the SDFs will become a target of guerrilla war," the motion said.
The opposition managed to delay debate on the bill from Thursday to Friday. It was sent to the Lower House by the Upper House Foreign Affairs and Defence Committee. Its approval is required before the Iraq bill can be enacted into law.
If enacted it will provide legal basis for the first dispatch of Japanese troops since World War II to a country where there is ongoing fighting.
The opposition, including the left-wing Social Democrats and the Communists, insists that the troop dispatch would violate Japan's anti-war constitution, put Japanese at risk and involve the country in the aftermath of an unjustifiable invasion.
Unnerved by mounting US casualties in Iraq, many Japanese are reluctant to see their troops -- who have not fired weapons in combat since the end of World War Two -- in danger.
The ruling coalition government, which enjoys a comfortable majority in the powerful chamber, is expected to defeat the opposition no-confidence motion in a vote later on Friday.
The government has pledged it would pass the Iraq bill before the current parliamentary session ends on Monday. Failure to do so would mean a huge loss of face for Koizumi possibly forcing him to call a snap general election.
But, the prime minister is determined to keep his promise to the US to send troops to Iraq despite the fact that a recent poll showed more than half of Japanese voters oppose the move.