Canadian criminal law allows for a common law defence of necessity. The leading case for the defence is Perka v. The Queen [1984] 2 S.C.R. 232 in which Dickson J. described the rationale for the defence as a recognition that:
However, it must be "strictly controlled and scrupulously limited." and can only be applied in the strictest of situations where true "involuntariness" is found.
Three elements are required for a successful defence :

1. the accused must be in imminent peril or danger
2. the accused must have had no reasonable legal alternative to the course of action he or she undertook
3. the harm inflicted by the accused must be proportional to the harm avoided by the accused

Each element must be proven on an objective standard. The peril or danger must be more than just foreseeable or likely. It must be near and unavoidable.
With regard to the second element, if there was a realistic or objectively reasonable legal alternative to breaking the law, then there can be no finding of necessity. Regarding the third element requiring proportionality, the harm avoided must be at least comparable to the harm inflicted.

In R. v. Latimer

R. v. Latimer

R. v. Latimer [2001] 1 S.C.R. 3, was a decision by the Supreme Court of Canada in the controversial case of Robert Latimer, a Saskatchewan farmer convicted of murdering his disabled daughter Tracy Latimer. The case had sparked an intense national debate as to the ethics of what was claimed as a...

(2001), the Supreme Court of Canada affirmed that the defense of necessity is not available to a defendant when (1) the killing occurred when there was no imminent danger to either the defendant or the victim, (2) reasonable legal alternatives are available besides killing, and (3) the harm inflicted is not in proportion to the harm avoided.

Classification of defence as excuse or justification

In Perka v. The Queen, the Court explores the history of the necessity defence in order to determine whether it is an excuse or a justification. The legal underpinnings of each are distinct. The majority concludes that under the Canadian Criminal Code, the defence of necessity excuses the accused of blame rather than acts as a justification of their actions. In a concurring opinion, Wilson J. leaves open the door to future case law finding that in some cases the defence can act as justification.

Judicial Antecedents

The Supreme Court previously discussed the defence of necessity in Morgentaler v. The Queen

Morgentaler v. The Queen

Morgentaler v. The Queen, [1976] 1 S.C.R. 616 is a famous decision of the Supreme Court of Canada where Henry Morgentaler unsuccessfully challenged the prohibition of abortion in Canada under the Criminal Code. The Court found the abortion law was appropriately passed by Parliament under the laws...

[1976] 2 S.C.R. 616 where they left open the possibility of its existence but did not conclude either way on the question.