Larose is the fourth Quebec MP to abandon the NDP since its fabled orange wave swept 59 of the province's 75 seats in the 2011 election.
The party's Quebec caucus chair, Robert Aubin, says Larose is betraying the memory of Jack Layton, who died a few months after leading the NDP to its best electoral showing ever.
Larose is joining Jean-François Fortin, a former Bloc Quebecois MP, to form Forces et Democratie, a party they say will be dedicated to defending the interests of Quebec's regions.
Fortin quit the Bloc last summer, after hard core sovereigntist Mario Beaulieu was chosen as the new leader.
NDP insiders say Larose quit because he was unlikely to win the nomination in his riding of Repentigny.
He was being challenged by a former staffer and a former candidate for the right to carry the NDP banner in the next election.
"This is a clear-cut act of disloyalty and betrayal of the memory of Jack Layton, along with the democratic choice of Repentigny voters," Aubin said in a written statement.
Aubin called on Larose to resign his seat and run for re-election under the new party banner, consistent with the principle he used to support as a member of the NDP.
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