Popular former Japanese Prime Minister Junichiro Koizumi said he was backing a fellow ex-premier in the race to become Tokyo governor, setting the stage for an electoral clash over nuclear power.

Morihiro Hosokawa, who served in the nation's top job in 1993-4 as head of a now-defunct coalition, announced his independent candidacy and scored a ringing endorsement from the influential Koizumi, with the two men both playing up their anti-atomic stance.

"I have made my decision to run in the Tokyo governor election," Hosokawa said on Tuesday. "I have a sense of crisis myself that the country's various problems, especially the nuclear issue, are matters of survival for the country."

The move positions him as the main alternative to the candidate backed by current premier Shinzo Abe and his pro-business Liberal Democratic Party (LDP), which wants to restart the nation's mothballed nuclear power plants.

Japanese voters have become wary of nuclear power since the tsunami-sparked disaster at Fukushima began in March 2011, but the issue failed to materialise in the national polls that brought Abe to power, with his opponents' apparent haplessness neutralising their anti-nuclear stance.

Tokyo's governor oversees a budget of some $60 billion and exercises control over the economic, political and cultural heart of Japan. The position is one of the most powerful roles in Japanese public life.

In addition to numerous powers to set policy, the Tokyo government is also a major shareholder in Tokyo Electric Power, the operator of the Fukushima plant, as well as several other nuclear power stations.

An anti-nuclear governor could use this position to curb the utility's drive to re-start its shuttered reactors.

Koizumi, whose flamboyant style and luxurious mane made him a popular prime minister between 2001 and 2006, has become an anti-nuclear convert in recent years, putting him at odds with his status as an LDP grandee.

"I will work hard and actively for Mr. Hosokawa's election," Koizumi told reporters.

"The election will be a battle between a group of people who say Japan cannot advance without nuclear power and another group of people who say Japan can."

Observers say with Koizumi's backing, Hosokawa, who celebrates his 76th birthday on Tuesday, is a serious challenger to the current favourite, former health minister Yoichi Masuzoe.

The post of Tokyo governor fell vacant last month when Naoki Inose stepped down in a money scandal after admitting he had been naive to accept an undeclared $500,000 from a hospital tycoon.

Inose, 67, had been one of the main faces of Tokyo's successful bid to host the 2020 Olympics.

A former air defence force chief with outspoken nationalist views, an anti-nuclear liberal lawyer and others have already thrown their hats in the ring.

Candidates have to formally declare themselves during office hours on January 23, and the vote takes place on 9 February.