Democratic Party lawmaker Shiori Yamao, a prominent adversary of Prime Minister Shinzo Abe in Diet debates, tendered her resignation from the party Thursday evening amid allegations of an extramarital affair, dealing a heavy blow to the nascent leadership of the struggling opposition.

"I'm deeply sorry my misleading behavior caused trouble to many people," a somber Yamao told a hastily arranged news conference in Tokyo.

"I've decided to leave the party to avoid bringing any confusion to the upcoming extraordinary Diet session."