Quebec Premier Rene Lavesque announced yesterday that the predominantly French-speaking province will decide in a referendum next spring whether to separate from the Canadian federation.

In making the announcement before the National Assembly, the province's legislature, at Quebec City, Levesque conceded that many in his separatist Parti Quebecois wanted the referendum scheduled for this fall.

But, he said, a spring vote would "best correspond to the interests of a people for whom that short day of decision will be so crucial."

The timing of the referendum, however, suggests that the separatist leaders have been shaken by a series of recent setbacks that they hope to regain their momentum by next spring.

Levesque won an overwhelming endorsement at a Parti Quebecois congress earlier this month to seek sovereignty for Quebec while keeping it in economic association with the rest of English-speaking Canada.

Under the party program, a sovereign Quebec would assume exclusive powers in the areas of legislation, taxation, foreign policy, internal security and defense. The program calls for a monetary union with English-speaking Canada, a free trade zone and common tariff policies.

Diplomatic observers said that apart from the recent political setbacks, the Parti Quebecois leaders are hoping that the new Canadian government of Conservative Prime Minister Joe Clark may make mistakes on the touchy national unity question that would play into their hands.

A public opinion poll two months ago showed that more than 50 percent of the people of Quebec were prepared to give Levesque a mandate to negotiate "sovereignty-association" with Ottawa and with other Canadian provinces.

Since then, however, the party has suffered a major setback in two provincial by-elections and its leadership was split over referendum policies for the first time since it came to power in October 1976.

New polls show that between 45 to 50 percent of Quebec's residents are supporting Levesque's "sovereignty-association" program, an indication that political difficulties have slowed the momentum of the separatists, who previously had been shown as steadily gaining strength.

Opposition to Levesque within the party first surfaced in late May when a senior party leader, Robert Burns, resigned from Levesque's Cabinet, saying that the party would lose both the referendum on the next provincial election.

At the party congress this month, these tensions surfaced again when Levesque's personal choice for the party's vice presidency was defeated by the candidate of militant separatists.

Another problem for the separatists is the emergence this year of a strong Liberal provincial leader, Claude Ryan, While Levesque believes that Quebec must become a sovereign state, Ryan argues that Quebec's position within Canada must be reinforced so as to have all the powers of a nation without seeking political independence.

Most French Canadians, who comprise almost one-third of Canada's 23 million people, have a strong attachment to their culture and language. But while the notion of sovereignty may seem attractive to them, they fear that Levesque's separatist drive will break up Canada and leave Quebec without economic benefits it currently enjoys. CAPTION: Map, no caption, The Washington Post