Japan's premier at the time of the Fukushima crisis said he was suing the current prime minister for defamation over online comments about the way the emergency was handled.
With less than a week to go before upper house elections, Naoto Kan, who is now in opposition, said on his official website he would be taking legal action against Shinzo Abe.
Kan's office said in the days immediately after a huge tsunami swamped Fukushima in March 2011, that his government pressed plant operator Tokyo Electric Power (Tepco) to use seawater to cool overheating reactors and prevent a catastrophe.
Tepco subsequently said Kan had wavered on allowing seawater to be used.
Kan's statement, posted yesterday, says Abe has repeated this claim. "Mr Shinzo Abe in his e-mail magazine ran a story entitled 'Mr Kan's instructions on using seawater (to cool reactors) are made up,' and despite my request for a correction and apology ... the story remains," Kan said on his official website.
Abe "is responsible for carrying out a fair election campaign ... I strongly demand he immediately admit his misconduct, delete the stories and apologise for this", Kan said.
Management of the crisis at Fukushima - the world's worst atomic disaster in a generation - has been picked over in the more than two years since the tsunami.
Last year, an independent panel on the disaster said Kan played a key role in preventing the crisis worsening. The panel said that as the situation on Japan's tsunami-wrecked coast deteriorated, Tepco wanted to abandon the plant and evacuate its workers, but that Kan ordered them to keep their men on site.
Experts concluded that if the premier had not stuck to his guns, Fukushima would have spiralled further out of control, with catastrophic consequences.
The utility did not co-operate with the study.
Kan's brief tenure as prime minister ended in September 2011. He has since become an anti-nuclear campaigner.
Japan goes to the polls on Sunday in an election for half the seats in the upper chamber. Abe's Liberal Democratic Party is expected to win comfortably after a campaign where nuclear power as an issue has been largely absent, displaced by talk of a rejuvenated economy.