Tokyo: Japan's centre-left ruling party is set to choose a new leader next Monday to replace unpopular incumbent Naoto Kan as the party president and therefore as the Prime Minister, officials said.

Kan, Japan's fifth Premier in as many years, has been expected for weeks to announce his resignation amid stinging criticism over his response to the March 11 quake and tsunami disaster and the Fukushima nuclear crisis.

Kan, 64, has since the Fukushima accident strongly advocated a nuclear-free future for Japan, a position that has put him at loggerheads with the conservative opposition and some members of his own party.

The August 29 election will be held on the condition that two bills, one of them to promote renewable energy, which Kan has championed, pass this week, said Democratic Party of Japan (DPJ) Secretary General Katsuya Okada.

The Parliament would most likely confirm the new Premier on August 30.

Finance Minister Yoshihiko Noda, 54, has been seen as the frontrunner to replace Kan, and Trade Minister Banri Kaieda, former Transport Minister Sumio Mabuchi and several others have also thrown their hats into the ring.

However, the focus is now on whether former Foreign Minister Seiji Maehara, the public's favourite according to opinion polls, will join the race.

A news agency cited "a lawmaker close to him" saying that Maehara, 49, had decided to stand, and reported that he would make an announcement on Wednesday.