The issue was raised after a referendum in 1995 saw Quebec within an inch of voting to quit Canada. The Parti Quebecois, the separatist party that has led the drive for sovereignty, argued then - and continues to do so - that if it won a referendum, it would have the right simply to cut loose and leave. This threat is partly aimed at gaining concessions from the rest of Canada in the event that Quebec wished to renegotiate an association with Canada.
The Supreme Court ruled yesterday that no such right of unilateral independence existed in Canadian law or international law. The court agreed that "the continued existence and operation of the Canadian constitutional order cannot remain unaffected by the unambiguous expression of a clear majority of Quebeckers that they no longer wish to remain in Canada". But, it said, "the primary means by which that expression is given effect is the constitutional duty to negotiate in accordance with ... constitutional principles".
The verdict will reinforce the view of many Quebecois that the federal institutions are set against them.
The court verdict creates a potentially dangerous situation. Quebec premier Lucien Bouchard, the PQ leader, is likely to press for a unilateral declaration of independence, which would put him outside the law. But the Canadian government probably would be unwilling to confront him over the issue, since it would create a stand-off that could only help him in a referendum.