Abe is widely expected to win another term as head of the Liberal Democratic Party with Ishiba likely to be his only challenger.

Polls suggest the premier is supported by around 70 percent of LDP members and winning the vote would put him on the path to becoming Japan's longest-serving prime minister ever.

Despite the odds, 61-year-old Ishiba said he had decided to challenge Abe to "restore confidence in the government, which should be at the service of citizens".

Earlier this year, Abe's popularity ratings slumped with his administration mired in two cronyism scandals.

But he has recovered from the rows, and is viewed as a shoo-in to win the internal election next month, which is held every three years.

In 2015 he ran unopposed, and in 2012 he beat four other contenders, including Ishiba, who was then seen as the front-runner.

Ishiba has expressed surprise at the dearth of challengers to Abe.

"It's a chorus of 'I won't run. I support Abe'," he told a local television station this week.

A defence hawk, Ishiba favours a revision of Japan's pacifist constitution along more radical lines than Abe, and has even controversially suggested Japan should consider having nuclear weapons.

He may well end up being Abe's sole rival, with another potential challenger, former foreign minister Fumio Kishida, already ruling out a bid.

Internal Affairs Minister Seiko Noda has reportedly considered running, but lacks support from enough party members.