The race to become the next governor of Tokyo kicked off Thursday in an election widely seen as a referendum on Japan's energy policy, almost three years after the nuclear disaster at Fukushima.

Observers say the election on February 9 will be a two-horse race between the anti-nuclear former prime minister Morihiro Hosokawa and Yoichi Masuzoe, an academic and former health minister, who served as a member of a Liberal Democratic Party (LDP) government.

"We have to stop (the policy of) restarting nuclear plants as soon as possible and adapt to a new era," Hosokawa said on the campaign trail Thursday.

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 swept Shinzo Abe to power, with his opponents' apparent haplessness neutralising their anti-nuclear stance.

The governor of Tokyo has no actual power to change national energy policy, but the sheer size of the city, with 13 million inhabitants and a pivotal place in the economic, political and cultural life of Japan, means its verdict will be tough to ignore.

High-profile backing

Hosokawa, whose 1993-4 premiership is little more than a footnote to modern political history, has the backing of wildly popular one-time prime minister Junichiro Koizumi.

The abundantly-coiffured Koizumi has shunned the limelight since his five-year premiership ended in 2006, but he emerged as an anti-nuclear convert midway through 2013 and began agitating for the permanent shuttering of Japan's nuclear reactors.

That put him at odds with current Prime Minister Abe, his one-time protege who has vowed to get the plants back on line when they have passed new, more stringent safety tests.

Popular memories of the 2001-2006 Koizumi premiership remain overwhelmingly positive, and his backing is expected to give Hosokawa a significant boost, say analysts.

"At this point Mr Masuzoe seems to be the strongest candidate as Mr Hosokawa has been largely mum about the details of his policy stances," said Sadafumi Kawato, professor of politics at Tokyo University.

However, "Mr Koizumi is still popular and honestly I can't predict the vote result," he added.

"But if Mr Hosokawa wins the election, it could be an obstacle to Prime Minister Abe's energy policy," he said, noting that Tokyo is a major shareholder of Tokyo Electric Power Co. (TEPCO), the operator of the Fukushima plant.

But in a note to clients, research consultancy Capital Economics said Abe, whose drive to reinvigorate Japan's sluggish economy is widely believed to be bearing fruit, may be able to ride out a negative result in the Tokyo poll.

"The influence of local politicians on energy policy is limited. The national government should therefore still be able to resume nuclear generation in coming months whichever way the Tokyo vote goes," it said.

Masuzoe, who gained fame as a political scientist and TV pundit, said his priority was to make the 2020 Tokyo Olympics a success.

"I want to have the best Olympics and Paralympics ever in history," he said, adding public safety, disaster prevention and social welfare services are also among his priorities.

On energy policy, he said "it is better to reduce the ratio" of nuclear power in energy consumption, adding "in (the) short term, securing the safety of nuclear plants is important".

At least 15 people filed their candidacy for the election, to be voted for by 10.82 million people, Tokyo's election board said Thursday afternoon.

Other candidates include Kenji Utsunomiya, 67, an anti-nuclear liberal lawyer backed by the Communist Party, and Toshio Tamogami, 65, a former air defence force chief with outspoken nationalist views.

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.

Prosecutors have begun quizzing him over the cash, reports said Thursday.