Québec Solidaire delegates have voted in favour of a proposal to merge with the smaller sovereigntist party Option Nationale.

"It's a huge step in the progressive sovereignist movement in Quebec," said QS co-spokesperson and MNA Manon Massé. "It's our family that's growing bigger, it's a movement that's expanding and taking a deeper root."

The vote was held Saturday morning behind closed doors at a party meeting in Longueuil, Que. Party officials said 80 per cent backed the merger.

The merger was supported by QS's leadership, including co-spokesperson Gabriel Nadeau-Dubois, but a grassroots movement had emerged to challenge the move.

Some members were concerned the party was making too many concessions to ON, such as adopting its harder line on sovereignty.

The merger proposal will be voted on by ON members later this month.

Québec Solidaire currently holds three seats in the National Assembly. ON doesn't hold any.