A day after the humiliating loss of his Diet seat in Sunday's general election, Banri Kaieda officially announced his resignation as president of the Democratic Party of Japan, the biggest opposition force.

"I deeply regret the lack of my ability" to retain a Lower House seat, Kaieda said during a news conference at DPJ headquarters in Tokyo. "When I traveled across the country, I heard many appeals from the public asking us to stop the Abe administration from getting out of control. But we couldn't win the (requisite) number (of seats)."

It is unprecedented for the head of a political party to lose a Diet seat in a national election, especially in view of the wide media exposure they get.