Claude Patry, who won the Jonquière-Alma riding as part of the NDP's "orange wave" in Quebec in the 2011 election, has left the party to join the separatist Bloc Québécois.
The surprise move drew a demand from Patry's former leader that he resign his seat to face a by-election as well as a withering comment from the prime minister.
Patry's decision comes as MPs prepare to vote on a BQ private member's bill to repeal the 2000 Clarity Act that set rules for future referenda on separation from Canada.
The NDP is opposing the bid to repeal the act. Patry told reporters Thursday he couldn't support the NDP's stance on the Clarity Act and its position on Newfoundland and Labrador's plan to develop the Muskrat Falls hydro-electric project, which has raised the ire of Quebec.
"Some things in life are non-negotiable," Patry said. "I will not get down on my knees."
In a statement in French released Thursday morning, Patry said he voted for sovereignty in the 1980 and 1995 referendums and that he still hopes Quebec will one day become a country.
NDP Leader Tom Mulcair told reporters on Parliament Hill Thursday that the NDP is a proudly federalist party and that the party's position on the constitution and Quebec is well known by all.
"In the 2011 election, every single [NDP] candidate, including Claude Patry, accepted the long-standing NDP position on the Constitution, which is called the Sherbrooke Declaration. Jack Layton and I met with him, we discussed this with him in detail, it was part of our policy," Mulcair said.
The 2005 declaration sets out the party's position on Quebec, including setting the bar for a referendum to separate from Canada at 50 per cent plus one.
"That is the position of our party, we're proud of it, we think it represents openness to Quebec. And you know what, that's why the Bloc Québécois only got 18.2 per cent of the vote in Jonquière-Alma in the last election and we were able to win convincingly. Claude Patry ran on that platform, he defended it, he's always believed in it."
Call for resignation
Mulcair also noted that Patry voted with the party last year on a bill that would force MPs who cross the floor to another party to resign their seat and run in a byelection and he called on Patry to do that now.
"We call upon [Patry] to have the courage of those convictions, to step down from his seat in Jonquière-Alama and run in a by-election if he thinks the people of his riding support him," Mulcair said.
Prime Minister Stephen Harper offered his own opinion on Patry's defection during a press conference in Rivière-du-Loup, Que.
"This is an issue that has concerned us for some time, does concern us, the ambiguity on national unity, on Canadian unity, that we have among some members of the NDP caucus in Quebec," Harper told reporters.
"We know that that caucus has many many links to Québec solidaire, which of course is a very strong sovereigntist party at the provincial level, and this phenomenon with the 'Bloc orange' should give everyone considerable pause, and I think that what has happened today is another example of this particular problem."
Patry's is the third departure from the NDP caucus since their 2011 breakthrough. Lise St-Denis, who won the Saint-Maurice–Champlain riding, defected to the Liberals in 2012. Thunder Bay MP Bruce Hyer quit the caucus to sit as an Independent over the party's position on the long-gun registry the same year.
The move brings the number of BQ MPs in the House of Commons to five.