TOKYO - A former foreign minister announced his candidacy to lead Japan on Tuesday just hours after unpopular Prime Minister Naoto Kan said he would be out of the picture by early next week.

Kan has been criticized for lacking leadership after the March 11 earthquake and tsunami and subsequent nuclear crisis, and survivors of the disasters complain of slow relief and recovery efforts. Polls show his approval rating is below 20 per cent.

Former Foreign Minister Seiji Maehara said Tuesday night he wants to take over the leadership to restore public trust and hope, especially among disaster victims.

"We must regain public trust in politics, and achieve policies that can help people sense of safety and hope for the future," Maehara said.

Maehara is a favourite to replace Kan and his entry could affect game plans for several other potential candidates.

A security expert and a China hawk, the 49-year-old lawmaker has warned against China's growing military spending and presence in the region.

Maehara was transport and foreign ministers during Tokyo's diplomatic spat with Beijing last year over small islands in the East China Sea claimed by both nations. China briefly suspended talks and some exports with Japan after a clash between Japanese patrol boats and a Chinese fishing boat and subsequent arrest of its captain. Maehara, from Kyoto, is an avid railway train fan and has actively tried to export Japan's "bullet train" technology.

Earlier Tuesday, Kan told Cabinet members that his days are numbered and they should be ready to resign en masse next Tuesday, according to Economy Minister Kaoru Yosano.

"He said each minister should do the utmost to prepare for a smooth handover and take care of pending businesses," Yosano told reporters.

Kan had promised in June to step down as soon as parliament passes two key bills -- related to budget and renewable energy each -- which it is set to do Friday. That would allow a leadership election Monday within Kan's ruling Democratic Party of Japan and a new prime minister -- Japan's sixth in four years. The party election campaign officially begins Saturday.

Japan's new leader would have to rebuild the country from the triple disasters, tackle a surging yen that is threatening the recovery and map out a new energy policy that is less reliant to nuclear power. A successor also would have to restore confidence on Japan's alliance with the U.S. Tokyo recently cancelled Kan's U.S. visit for talks with President Barack Obama, expected in early September, due to the political uncertainty.

The ruling Democrats have also struggled with a hung parliament due to their defeat in the upper house elections last year and a party infighting.

Maehara said the ruling lawmakers must be as one and make a fresh start to grapple with the difficult times.

"It is our political responsibility to overcome this national crisis as we stand by the disaster victims," he said, calling for support for him. "With your co-operation and participation, we can make a vigorous Japan."

Maehara resigned just before the March disasters over an illegal political donation, but he is a favourite to replace Kan in terms of public support and party power base. Other candidates are Finance Minister Yoshihiko Noda, Trade Minister Banri Kaieda and Agricultural Minister Michihiko Kano.

Maehara had earlier indicated supporting Noda instead of entering the race, apparently changed his mind because of notion that Noda, who has voiced the need for a tax increase as finance minister, would have trouble gaining voter support, according to Japanese media reports.

Before his candidacy announcement Tuesday, Maehara met with a longtime party supporter and influential business leader to reveal his intention to run and seek support.

"I just said go for it," Kazuo Inamori, founder of electronic component maker Kyocera Corp. and a long-standing supporter of the Democrats, told reporters after meeting Maehara.