The Supreme Court of Canada has rejected Conservative government arguments that Parliament should be able to make unilateral changes to the Senate.
Prime Minister Stephen Harper had suggested that changes like term limits and Senate candidate elections would be housekeeping measures to be passed by Parliament.
But the high court has ruled that such changes would need constitutional amendments that would require the approval of at least seven provinces with at least 50 per cent of the population.
Senate abolition, which the Prime Minister had threatened if his reforms were rejected, would require the unanimous consent of all provinces.
The decision throws a wrench into the Prime Minister's plans to reform the upper chamber.
Harper has already said he will not open constitutional negotiations with the provinces.