Mitzna is expected to officially announce the move in a press conference set for Sunday.
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Mitzna is the first top political figure to join the former Kadima chairwoman's new movement.
Hatnua is racing to attract more MKs to its roster. Livni is trying to recruit Kadima MKs in particular. Six have joined her so far, and once she recruits a seventh Kadima could file a motion to split the party.
Such a move would enable Livni's new party to receive funding – according to the number of "defecting" Knesset member who join her ranks.
MK Yoel Hasson, who supported Livni in her bid for a second term at Kadima's helm, is overseeing the potential spilt motion, which has so far been backed by MKs Orit Zuaretz, Rachel Adatto, Majalli Whbee, Shlomo (Neguse) Molla, Robert Tibayev and Hasson himself.
Sources close to Livni said that Hatnua could "easily attract more than seven MKs, but at the end of the day we would prefer someone who will bring added value to the party."
Labor issued a statement following Mitzna's decision, saying: "Amram Mitzna's bid to run for Labor chairman a year ago failed miserably, as he came in last.
"It's a shame that he chose to make this move after thousands of Labor members celebrated democracy by voting for their representatives.
"We regret that people who supposedly stand for 'clean politics' dismiss their voters' choice. This move only plays into Netanyahu's hands," the party's statement said.
Yoval Karni is a Ynet and Yedioth Ahronoth correspondent
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