Supreme Court of Canada
Chief Justice: Antonio Lamer Puisne Justices: Claire L'Heureux-Dubé, John Sopinka, Charles Gonthier, Peter Cory, Beverley McLachlin, Frank Iacobucci, John C. Major, Michel Bastarache
Unanimous reason by: The Court Reference re Secession of Quebec, [1998] 2 S.C.R. 217 was an opinion of the Supreme Court of Canada regarding the legality, under both Canadian and international law, of a unilateral secession of Quebec from Canada.
Both the Quebec government and the Canadian government stated that they were most pleased with the Supreme Court's opinion, pointing to different sections of the ruling.
As expected by many jurists familiar with the subject, the Supreme Court answered "No" to the first two questions.

Background
The Governor in Council (effectively, the Cabinet of Canada) submitted the request for an advisory opinion on the following three specific questions:

Under the Constitution of Canada, can the National Assembly, legislature or government of Quebec effect the secession of Quebec from Canada unilaterally?
Does international law give the National Assembly, legislature or government of Quebec the right to effect the secession of Quebec from Canada unilaterally? In this regard, is there a right to self-determination under international law that would give the National Assembly, legislature or government of Quebec the right to effect the secession of Quebec from Canada unilaterally?
In the event of a conflict between domestic and international law on the right of the National Assembly, legislature or government of Quebec to effect the secession of Quebec from Canada unilaterally, which would take precedence in Canada? Questions addressed
There were an unprecedented 15 interveners. However, the Quebec government refused to take part and was not represented. In its place the Court appointed André Jolicoeur as an amicus curiae to argue for the sovereigntist side.
The federal government's submission argued that the only way a province could secede from Canada would be through a constitutional amendment. Only an amendment through section 45 would allow for unilateral amendments. However, that section does not apply here. To attempt to secede unilaterally would violate the constitution on two grounds. First, it would violate the rule of law by ignoring the authority of the constitution as supreme law of the country, and second, it would violate Canadian federalism by acting with powers only allocated to the federal government.
The amicus curiae's submission argued several points. First, they argued that the reference was invalid; the question is purely a political one and thus is outside the authority of the Court to answer under section 52 of the Supreme Court Act. They attempted to analogize the use of the US political question doctrine to the Canadian constitution. Furthermore, the question is speculative and premature as there are no substantive facts at question. Second, they focused on the second question, claiming that the ability to separate comes from international law. They argued that the "peoples of Quebec" had a right to self-determination under the Charter of the United Nations and thus can secede given the consent of a majority of the Quebec peoples. They further claimed that since there is no international law barring separation then by convention there must be an implied right to do so. Their primary argument was that the doctrine of effectivity gave them authority to secede. That is, recognition of a new state by other countries would validate the separation. They further claimed that the doctrine of effectivity is part of the constitutional convention through its practice in other parts of the commonwealth.
Several aboriginal interveners submitted facta on their right to stay in Canada based on aboriginal treaties and their right to self-determination, further noting that they have already held two referendums where they decided against the separation of the aboriginal peoples from Canada. Their factum attacked the Attorney General's factum on the basis that they completely ignored the role of the aboriginal people within the constitution.

Submissions

The opinion
The court addressed the three questions in order. First, they stated that under the Canadian Constitution (and with Quebec being a party to it since its inception), unilateral secession was not legal. However, should a referendum decide in favour of independence, the rest of Canada " would have no basis to deny the right of the government of Quebec to pursue secession." Negotiations would have to follow to define the terms under which Quebec would gain independence, should it maintain that goal.

Right to secede under Canadian law
The answer to the second question, which concerned Quebec's right under international law to secede, gave the opinion that the international law on secession was not applicable to the situation of Quebec. The court pointed out that international law "does not specifically grant component parts of sovereign states the legal right to secede unilaterally from their 'parent' state."
The Supreme Court of Canada's decision stated that the right of a people to self determination was expected to be exercised within the framework of existing states, by negotiation, for example. Such a right could only be exercised unilaterally under certain circumstances, under current international law. The court held that:
The various international documents that support the existence of a people's right to self-determination also contain parallel statements supportive of the conclusion that the exercise of such a right must be sufficiently limited to prevent threats to an existing state's territorial integrity or the stability of relations between sovereign states.
and that
A state whose government represents the whole of the people or peoples resident within its territory, on a basis of equality and without discrimination, and respects the principles of self-determination in its own internal arrangements, is entitled to the protection under international law of its territorial integrity.
The court stated in its decision that under international law, the right to secede was meant for peoples under a colonial rule or foreign occupation. Otherwise, so long as a people has the meaningful exercise of its right to self-determination within an existing nation state, there is no right to secede unilaterally.
For close to 40 of the last 50 years, the Prime Minister of Canada has been a Quebecer. During this period, Quebecers have held from time to time all the most important positions in the federal Cabinet. During the 8 years prior to June 1997, the Prime Minister and the Leader of the Official Opposition in the House of Commons were both Quebecers. At present, the Right Honourable Chief Justice and two other members of the Court, the Chief of Staff of the Canadian Armed Forces and the Canadian ambassador to the United States, not to mention the Deputy Secretary-General of the United Nations, are all Quebecers. The international achievements of Quebecers in most fields of human endeavour are too numerous to list. Since the dynamism of the Quebec people has been directed toward the business sector, it has been clearly successful in Quebec, the rest of Canada and abroad.
The Supreme Court further stated that: Quebec could not, despite a clear referendum result, purport to invoke a right of self-determination to dictate the terms of a proposed secession to the other parties to the federation. The democratic vote, by however strong a majority, would have no legal effect on its own and could not push aside the principles of federalism and the rule of law, the rights of individuals and minorities, or the operation of democracy in the other provinces or in Canada as a whole.

Rights to secede under international law and self-determination
Since the court saw no conflict between Canadian law and International law on the question (neither would allow Quebec to secede unilaterally), it considered it unnecessary to answer the question.

Reaction of the federal government

List of Supreme Court of Canada cases (Lamer Court)
Unilateral Declaration of Independence
Politics of Canada
Politics of Quebec
1980 Quebec referendum
1995 Quebec referendum
Timeline of Quebec history
Timeline of Canadian history