Three men will challenge Yoshihiko Noda for leadership of the governing Democratic Party of Japan, with the Prime Minister widely expected to keep his job.

Under party rules, there must be an election every two years for the job, which currently comes with the post of Prime Minister, but after his only serious rival ruled himself out, Noda's grip looks secure.

As prime minister and leader of the DPJ - presently the biggest party in parliament - Noda has pushed through unpopular legislation on doubling sales tax.

This, alongside general disenchantment with his once-popular party, has left many lawmakers fearing for their jobs in the general election that is expected this autumn.

Noda's telegenic environment minister had been seen as a credible challenger until he ruled himself out of the running last week.

Goshi Hosono would have been the seventh prime minister in six years, and Japan's youngest ever head of government, if he had won.

Noda will face challenges from three of his backbenchers: former internal affairs minister Kazuhiro Haraguchi, former agriculture minister Hirotaka Akamatsu, and another former agriculture minister Michihiko Kano.

Observers say the factionally-divided DPJ is likely to suffer at the hands of voters disappointed by their lacklustre three years in office.

The DPJ came to power in 2009 after five decades of almost unbroken rule by the Liberal Democratic Party (LDP). But its once-radical agenda was largely jettisoned.

The LDP is also in the throes of its own leadership tumult, with incumbent Sadakazu Tanigaki today saying he would not run for re-election.

DPJ members vote on September 21. The LDP ballot will come five days later.

Over the weekend Toru Hashimoto, the populist governor of Osaka, began in earnest his push onto the national political scene, when he announced the name of the party that will put up candidates in the general election.

- AAP