Ryuichi Yoneyama was elected governor of Niigata prefecture in 2016 on a pledge to prevent the restarting of the Kashiwazaki-Kariwa power station, the world's biggest nuclear plant, about 200 kilometres (130 miles) northwest of Tokyo.

His unexpected victory, in which he narrowly beat a government-supported candidate, posed a challenge for the pro-nuclear policy of Prime Minister Shinzo Abe.

Yoneyama, a 50-year-old unmarried doctor and lawyer, paid women in their 20s to have sex, according to the Shukan Bunshun weekly magazine.

"I decided to step down to avoid further turmoil and to take responsibility for my actions," he told reporters, bowing deeply in front of the cameras.

"It was hard to find someone to date... I did give gifts and money to get attention" from the women, he said.

"I wasn't able to tackle the nuclear issue, which I thought was a historic mission."

A 22-year-old student told the magazine he was "a good client."

Prostitution is illegal in Japan but prosecution is rare.

There are seven reactors across the 4.2-million-square metre (45-million-square feet) Kashiwazaki-Kariwa site.

The central government can overrule a governor's opposition to restarting nuclear reactors.

But Abe has promised to win approval from local communities before approving restarts under stricter post-Fukushima safety rules.

Dozens of reactors across Japan were switched off in the aftermath of the March 2011 Fukushima accident, the worst nuclear disaster in a generation, and there are seven currently operating.

The catastrophe forced resource-poor Japan to turn to expensive fossil fuels to plug its energy gap.

But fears about the safety of nuclear power and radiation exposure linger, challenging a push by Abe and utility companies to switch back on the country's stable of reactors.