https://en.wikipedia.org/wiki/Intervention_(law)

In general, it is within the discretion of the court to allow or refuse an application to intervene. There are exceptions to this however (for example, under subrule 61(4) of the Rules of the Supreme Court of Canada, if the court has stated a constitutional question then the attorney general of any province or territory, or of the federal government, may intervene "as of right", i.e. without the need to be granted leave to intervene).

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