Prime Minister Pierre Trudeau has decided to counter the threat of Quebec secession by seeking legislation that would allow the federal government to conduct a referendum in the province should a majority of its citizens vote for independence next year.

Marc Lalonde, minister of federal-provincial relations and one of Trudeau's close associates, disclosed that he would introduce the measure in parliament next month.

"We want to have this weapon in our arsenal but we hope we won't have to use it," he said.

The government will also introduce a constitutional reform package that would give Canada's 10 provinces a greater say in the appointments of Senate and Supreme Court members.

But the "referendum weapon" was seen by Trudeau's associates as the ultimate deterrent against the separatist government of Quebec Premier Rene Levesque.

Levesque has vowed to hold a provincewide referendum on independence next year, seeking a mandate from its predominantly French-speaking citizens to separate Quebec from the rest of Canada.

Trudeau and his colleagues, however, believe that the people of Quebec will not be given a clear-cut choice. Levesque has been somewhat ambiguous on the issue, arguing that Quebec was to gain political independence without disturbing the existing economic arrangements.

Hence the need for a hard, clean choice, Lalonde explained, that would make it clear to the voters that independence could not be achieved without cost: Confronted with an unambiguous question of whether they want to get out of Canada, most would chose to stay in, Lalonde said.

Diplomatic sources here indicated that the polls have consistently showed that roughly 20 percent of the people of Quebec are hard-core separatists and that another 30 percent are sympathetic to Levesque's notion of political independence and continued economic association with the rest of Canada.

Given the sizable undecided vote, it is conceivable that Levesque could win a mandate to negotiate independence if the voters are faced with an ambiguous plebiscite question.

Much of the maneuvering during this week's economic conference of top federal and provincial officials centered on Levesque's efforts to display a cooperative spirit in dealing with the provincial premiers while seeking confrontation with Trudeau.

Traudeau, in turn, took Levesque head on since the question of national unity and Trudeau's commitment to federalism was expected to be the key issue in the next general election.

In contrast to the United States where state governments have become weaker, the dynamics of Canadian politics has led to a movement of power to the provinces. One example is that the provinces now control 60 per cent of all public spending.

Since Levesque's separatist Party Quebecois was elected in 1976, the threat of Quebec separatism has become a major national issue, creating widespread uncertainties about Canada's future.

The angry confrontation between Trudeau and Levesque carried live on national television this week was seen here as a prelude to both federal elections and the Quebec referendum.