By Randall Palmer

OTTAWA, Feb 20 (Reuters) - Quebec's two main opposition parties said on Thursday they would oppose the separatist Parti Quebecois government's budget, assuring a spring election, and raising the possibility of an eventual third referendum on independence from Canada.

The Parti Quebecois currently heads a minority government, and has a comfortable lead in the polls. It has signaled it might well call an election on its own in the next two weeks, before it is brought down by a vote on its budget. In either case, an election by mid-April is likely.

If Premier Pauline Marois decided not to call an election on her own, her Parti Quebecois would need the support of either the Quebec Liberals or the Coalition Avenir Quebec (CAQ), but both have said they oppose the budget.

"It will be impossible for us to support the budget," Quebec media quoted Liberal leader Philippe Couillard as saying.

CAQ spokesman Jean-Francois Del Torchio said: "We'll be voting against if there is a vote."

However, he added that Marois had pledged not to be defeated in the legislature, suggesting she might call an election.

The Parti Quebecois, which would like to take the mainly French-speaking province out of Canada, won a minority of seats in the 2012 election with just under 32 percent of the vote.

An online panel survey by the polling firm CROP, published on Tuesday by La Presse newspaper, put the Parti Quebecois at 40 percent. The survey had the Quebec Liberals at 34 percent and the Coalition Avenir Quebec at 16 percent.

La Presse columnist Vincent Marissal said that, with 40 percent support, and 47 percent among francophones, the Parti Quebecois had a "passport to a majority" government.

It needs to have a majority of seats in the Quebec National Assembly, the provincial legislature, to call a referendum on independence.

The Parti Quebecois lost similar referendums in 1980 and 1995, the latter by just over one percentage point. It has signaled it would not automatically call a third referendum, but that possibility will always loom in the background.

The Canadian dollar and bonds suffered during the 1980 and 1995 campaigns.

Thursday's budget, which will serve more as a Parti Quebecois platform, projected deficits of C$2.50 billion ($2.27 billion) for 2013-14 and C$1.75 billion for 2014-15, with a balanced budget in 2015-16, unchanged from the projections the government gave in November.