Japan's Prime Minister Naoto Kan on Thursday offered to step down once the country has recovered from the March 11 quake and tsunami in a concession to his enemies ahead of a no-confidence vote.

The centre-left leader, in power just under a year, was seeking to quell a revolt within his own party ranks that put his future on a knife-edge.

Kan is Japan's fifth premier in as many years, and his ouster would perpetuate the revolving-door leadership as the world's number-three economy struggles with disaster recovery, a flagging economy and huge public debt.

The opposition conservative Liberal Democratic Party (LDP) -- which was ousted in 2009 after more than half a century of almost unbroken rule -- submitted a no-confidence motion late Wednesday, backed by two small parties.

Kan's enemies face a tough battle, but observers saw the contest as too close to call as frantic lobbying and bullying of lawmakers continued.

The parliamentary debate was due to start after a delay around 0430 GMT, with vote results expected about two hours later.

Hours before the vote, Kan said he would hand over power at an undefined future date, once the main challenges of rebuilding from the quake, tsunami and nuclear disaster are substantially met, but asked lawmakers to support him now.

"Once my handling of the earthquake disaster is settled to some extent and I have fulfilled my role to some extent, I would like younger generations to take over (my) various responsibilities," Kan, 64, told parliamentarians in his Democratic Party of Japan (DPJ).

"Let me fulfil my responsibilities until we see the (reconstruction) work near settlement. Let me meet this responsibility with all of you.

"For that purpose I sincerely urge members of the DPJ and the lower house to unite and reject the no-confidence motion tabled by opposition parties today."

LDP leaders have angrily accused Kan of bungling the response to the disaster that left more than 23,000 dead and missing and 100,000 still in shelters and triggered the world's worst nuclear crisis since Chernobyl in 1986.

Kan's DPJ has a majority in the 480-seat Diet lower house, but he could still lose the vote because a powerful rival and faction boss, Ichiro Ozawa, has said he would vote against Kan.

In order to push through the no-confidence motion by a majority vote -- which would force Kan to either step down or call fresh elections -- the LDP would need the backing of more than 80 DPJ rebel lawmakers.

The ruling party has fought hard to maintain party discipline, threatening lawmakers hostile to Kan with expulsion if they vote against him or abstain.

The rebellion's scandal-tainted leader, one-time LDP heavyweight Ozawa, has earned the nickname of "the destroyer" over his four-decade political carrier for his record of forging and tearing down alliances.

Ozawa last year narrowly lost a leadership challenge against Kan, and a shadow hangs over his future after his indictment over political funding irregularities, in which he denies any wrongdoing.

Ozawa on Wednesday night held a meeting of 70 lawmakers, many of whom were expected to vote against Kan, and predicted the motion would pass.

Former premier Yukio Hatoyama earlier said he opposed Kan but, after the prime minister's comments Thursday, suggested he would back him, saying that the current crisis called for unified action and DPJ party unity.

Among smaller parties, the Buddhist-backed New Komeito and conservative Sunrise parties jointly submitted the no-confidence motion with the LDP.

The Social Democrat and Communist parties were expected to abstain, while the People's New Party and some independents may back Kan for now.

The political infighting at a time of crisis has disgusted many Japanese.

The Asahi Shimbun daily said in an editorial that the Diet's duty is to make laws and a budget to speed disaster recovery and said "we feel strong resentment at the lawmakers who are brazenly playing power games."