Renzi resigned after after Italians overwhelmingly rejected a package of reforms in a December 4th ballot that became a vote on his own record.
"We lost, we ultra-lost," the 41-year-old Renzi told lawmakers from his centre-left Democratic Party (PD).
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"I made the mistake of politicizing the referendum: I didn't see it happening and I was wrong."
"We lost among young people, particularly the 30 to 40 year olds. We lost at home and that hurts."
Renzi's replacement as prime minister, Paolo Gentiloni, had his new government endorsed on Wednesday and hopes to guide Italy to elections in early 2018, but there are suggestions polls could come up to a year early.