The opposition offensive came as reports said the formal order for a full deployment would come as early as Monday next week.
"Sending the Self Defence-Forces (SDF) to a battlefield is like trampling on the (fundamental) principle of the Constitution," Naoto Kan, head of the nation's largest opposition bloc, the Democratic Party of Japan, said on Wednesday at the start of a three-day parliament session.
"Prime Minister Koizumi, who gave an order for activities which contravene the current Constitution, lacks the qualifications for prime minister," Kan told parliament. "We want to strongly demand his resignation."
Japan has already dispatched three advance teams to Kuwait and the southern Iraqi city of Samawa before its planned deployment of the core contingent of troops, in the first deployment by Japan's military in a hostile region since World War II.
The country's post-war constitution bans the use of force in settling international disputes and the deployment plan has aroused fears among neighbouring nations about a militarist revival in the country.
Defending the troops
Koizumi defended his decision to send troops to Iraq, saying: "The SDF will carry out humanitarian assistance in non-combat areas. Legitimate self-defence is totally different from the use of force under a state order."
"Prime Minister Koizumi, who gave an order for activities which contravene the current Constitution, lacks the qualifications for prime minister . We want to strongly demand his resignation"
Head of the Democratic Party
He said the Japanese troops would withdraw from Samawa if "a battle" takes place in the city, without elaborating.
News reports said on Wednesday Koizumi planned to formally order the defence agency to send main units of the Ground Self-Defence Force (army) to Iraq as early as Monday next week.
Koizumi will be briefed about security conditions in Samawa by
some members of the advance team who will return to Japan on Friday, the Asahi Shimbun and Jiji Press said, quoting government sources.
Koizumi would then give the green light for deployment after
notifying his coalition ally, the New Komeito party, which is backed by a large lay-Buddhist organisation and has been cautious about the deployment.
As a first step, 80 ground troops are to fly to Samawa by the
end of January to begin constructing a basecamp, they said.
Japan is then to send 440 personnel during February and March so that the troops can begin providing humanitarian and reconstruction support, including work on the water supply.