Defence of Necessity
The common law provides for a defence of necessity (sometimes called "duress of circumstances") for "emergency situations where normal human instincts, whether of self‑preservation or of altruism, overwhelmingly impel disobedience." The defence provides a legal excuse (as opposed to a justification) for conduct making out the offence.
- Purpose of Defence
The key basis for the common law defence is the "criterion" of "moral involuntariness of the wrongful action". Involuntariness is measured against "society's expectation of appropriate and normal resistance to pressure".
To be truly involuntary. The act must be "inevitable" and "unavoidable".
There is no need for an accused to have clean hands to engage the defence. Negligence or involvement in a criminal or immoral activity is not a bar to the defence.
However, the defence will not be available where "actions or circumstances which indicate that the wrongful deed was not truly involuntary" or where there exists a reasonable legal alternative..
- Limited Scope
The defence is to be "strictly controlled and scrupulously limited" to situations of true involuntariness.
- Burden of Proof
The accused has the burden to establish an air of reality that the defence applies. This requires at least some evidence that all the elements of necessity may be satisfied. Once established the burden moves to the Crown to establish beyond a reasonable doubt that at least one of the elements of the defence does not apply.
R v Perka, 1984 CanLII 23 (SCC), per Dickson J
R v Ruzic, 2001 SCC 24 (CanLII),  1 SCR 687, per LeBel J, at para 68
- Perka, ibid.
R v McKay, 1992 CanLII 1952 (BCCA), per Taylor JA
R v Latimer, 2001 SCC 1 (CanLII), per curiam, at paras 26 to 34
- Perka, supra, at p. 404 (cited to CCC) see also Air of Reality Test
The elements to make out the defence of necessity requires proof that:
- the accused must be in imminent peril or danger;
- the accused must have had no reasonable legal alternative to the course of action he or she undertook; and
- the harm inflicted by the accused must be proportional to the harm avoided by the accused.
The elements of imminent peril and no alternative is determined on a modified objective standard taking into account the situation and characteristics of the accused. This involves, the trier-of-fact determining whether the accused's "perception of his situation, and of the absence of any lawful alternatives, had an objectively reasonable foundation".
The element of proportionality is measured on an objective standard.
- See R v Morgentaler v The Queen, 1975 CanLII 8 (SCC),  1 SCR 616, per Pigeon J, at p. 678
R v Latimer, 2001 SCC 1 (CanLII), per curiam, at paras 32 to 34 (“The accused person must, at the time of the act, honestly believe, on reasonable grounds, that he faces a situation of imminent peril that leaves no reasonable legal alternative open.”)
R v Nelson, 2007 BCCA 490 (CanLII), 228 CCC (3d) 302, per Rowles JA, at para 32
Nelson, ibid., at para 39
Latimer, ibid., at para 34
Imminent Peril or Danger
The peril or danger must be more than just foreseeable or likely. It must be near and unavoidable. At the least, "the situation must be so emergent and the peril must be so pressing that normal human instincts cry out for action and make a counsel of patience unreasonable."
The consequence must be "imminent or harm unavoidable and near... It must be on the verge of transpiring and virtually certain to occur."
Necessity "may be applied only in truly emergent circumstances, and only when the person at risk has no alternative but to break the law." It cannot apply to conduct that had "been specifically enjoined", nor can it "operate to avoid a peril that is lawfully authorized by the law".
No Reasonable Legal Alternative
The question of reasonableness turns on the particular circumstances.
Necessity requires that compliance with the law be "demonstrably impossible"
The inquiry in the second requirement focuses on whether the accused had any real choice. Where he has no real choice then his actions are effectively not "morally voluntary".
The accused "need not be placed in the last resort imaginable, but he must have no reasonable legal alternative."
The judge need not consider every potential possibility in hindsight.
In an impaired driving case, it has been accepted that the accused need not knock on the doors of strangers late at night during a storm to find someone who would drive them.
R v Hibbert, 1995 CanLII 110 (SCC),  2 SCR 973, per Lamer CJ, at para 53
- R v SRM, 2010 SKPC 93 (CanLII), per Tomkins J, at para 44
R v Latimer, 2001 SCC 1 (CanLII), per curiam, at para 30
- R v Costoff, 2010 ONCJ 109 (CanLII), per Bourque J, at para 26 cited with approval in R v Murray, 2010 ABQB 784 (CanLII), per Verville J, at para 35
- Costoff, supra, at para 24
To be proportional, the harm avoided does not need to "clearly outweigh" the harm inflicted.  The harm avoided must only be of "comparable gravity" to the harm inflicted. Or alternatively, it must be "less than" the harm that was sought to be avoided.
In considering this element on an impaired charge. The harm associated with the offence is grave. The fact that no one is hurt is of little importance. The comparator harm avoided should relate to the "preservation of life".
R v Latimer, 2001 SCC 1 (CanLII), per curiam, at para 31
Latimer, ibid., at para 31
R v Perka, 1984 CanLII 23 (SCC),  2 SCR 232, per Dickson J, at p. 253 ("I would therefore add to the preceding requirements a stipulation of proportionality expressable, as it was in Morgentaler, by the proviso that the harm inflicted must be less than the harm sought to be avoided.")
R v Desrosiers, 2007 ONCJ 225 (CanLII),  OJ No 1985 (ONCJ), per Keast J, at para 31
R v Costoff, 2010 ONCJ 109 (CanLII),  OJ 1261, per Bourque J, at para 29
An accused's contributary fault will also disentitle them from using the defence. Where the circumstances causing the offence "was clearly foreseeable to a reasonable observer" and the accused "contemplated or ought to have contemplated that his actions would likely give rise to an emergency requiring the breaking of the law", then their actions would not be involuntary.
Specific Types of Offences
Necessity was not available for an accused who burned a highway bridge in order to draw attention to its need for repair as there were reasonable alternatives.
- R v Stevenson,  5 WWR 737, 42 Man.R. (2d) 133 (Man. Q.B.)(*no CanLII links) leave denied,  1 WWR 767 (Man. C.A.)
Where the accused or someone in his protection is at "immediate risk of physical harm, if no reasonable alternative is available and, if the driving is for no longer than is necessary to escape the harm, the defence of necessity will succeed".
Necessity no longer applies where the driver drove longer than is necessary in the circumstances.
- R v LS, 2001 BCPC 462 (CanLII),  BCJ No 3062, per Bayliff J, at para 25
R v Drake, 1998 CarswelMan 232 (Prov.Ct.)(*no CanLII links)
- the accused drove past a police station, hospital and gas station
- drove past a police station
- the accused drove past a police station, hospital and gas station
Necessity is a potential defence for dangerous driving causing death where the accused had reason to flee from gunshots that were putting his life at risk.
Break and Enter
Necessity has been considered in relation to break and enter. 
Fraud and Related
A woman was found not guilty of fraud for failing to report her co-habitation to social services due to suffering from battered wife syndrome and believed she had no other choice.  Battered wife syndrome was not successful in a necessity defence for possession and laundering proceeds of crime.
False pretenses not available where there are other options available.