Quebec's long-awaited independence referendum that will test the survival of a united Canada will be held May 20, the Quebec provincial government announced today.

The choice of that date, the earliest possible, reflects a widespread belief that the pro-independence forces of Quebec Premier Rene Levesque have gained a substantial advantage in recent weeks in the campaign leading to the referendum.

In the vote, the residents of the predominantly French-speaking province will be asked to give Levesque authority to negotiate "a new agreement" to transform Quebec into a sovereign state in "economic association" with the rest of Canada.

Levesque is pledged to seek public approval for changes in Quebec's status "resulting from these negotiations." But a separatist victory in the referendum would set off a chain of events that could reshape the map of the country.

The French separatist drive for an independent state is one of the most significant political conflicts in North America since the United States was split over the slavery issue. Prime Minister Pierre Trudeau has said he would not flinch from "using the sword" to maintain Canadian unity.

In his first major speech since he was elected to his fourth term as prime minister, Trudeau today threw the full weight of his prestige behind the forces opposing Levesque's drive for independence.

Trudeau told the House of Commons that there is "no question" of discussing independence with Quebec because the province is not an independent country.

Trudeau also said that many of the premiers of Canada's 10 provinces have publicly stated that they have no intention of engaging in similar negotiations with Levesque.

Besides saying that the issue of Quebec's future must be discussed in Parliament, Trudeau also said he would campaign against Quebec independence in the province in coming weeks. Senior members of his Liberal party, worried that Levesque was gaining a lead in the pre-referendum campaign, have recently urged Trudeau to take a strong stand in the debate.

In Quebec City, Levesque told the provincial legislature today that the campaign will last a minimum 35 days permitted by law.

Levesque's low-key approach to independence has been instrumental in the apparent upsurge of support for his Separatist Parti Quebecois referendum proposal. Trudeau today again accused Levesque of presenting Quebec residentns with an ambiguous choice.

"If they had asked a clear question to the Quebecois, 'Do you want independence or not?' the Quebecois would have answered a resounding 'no,' said Trudeau.

Levesque, in announcing the referendum date, said its outcome would be "decisive" for Quebec's future. He said there was a "surge of solidarity" across the province and he appealed to non-French groups to join "the mainstream of solidarity" so that they may share in the victory he is confident of obtaining on May 20.

The latest public opinion poll conducted in Quebec showed 48 percent of those questioned support Levesque, 43 percent are opposed, with 9 percent undecided.

Of Quebec's 6.3 million population, 80 percent are French-speakers. The English-speaking minority is solidly opposed to the sovereignty drive.