Necessity is a complete defence to any offence, and has three elements: Re A (Conjoined Twins)  2 WLR 480.
- The defendant’s actions were necessary to avert an irreparable and inevitable evil;
- The defendant did not do more than was reasonably necessary to avert that evil; and
- The defendant’s actions and their consequences were not disproportionate to the evil the defendant was trying to avert.
It appears that necessity is not available to avert all evils.
The courts have denied that necessity would be available to a homeless person who committed breaking and entering for shelter or theft for food, for example: Southwark London Borough v Williams  2 All ER 175.
Most successful necessity cases involved a threat to life or potential serious injury. The exception is medical treatment cases, where necessity has successfully been invoked for lesser threats to physical and mental health: Gillick v West Norfolk and Wisbech Area Health Authority  AC 112.
The ‘Selection’ Problem
If the victim died, the defence of necessity only applies if there is no ‘selection problem’: Re A (Conjoined Twins)  2 WLR 480. A selection problem is where multiple people in the scenario could die, which could be averted by killing any one of them: R v Dudley and Stephens (1884) 14 QBD 273. Necessity only applies if there is no need to select a victim between multiple options, as such. For example:
Scenario 1: Three men are trapped in a boat lost at sea. All three will starve unless they kill and eat one of them. Any of the men are a viable candidate to be the victim. As such, there is a selection problem. The defence of necessity cannot apply.
Scenario 2: The defendant is a mountain climber who is roped to another person. That person slips and falls. They will drag the defendant down with them (killing both) unless the defendant cuts the rope. There is only one option of who to kill to avert the evil, so no selection problem. Necessity is available.
Necessity and Negligence
Necessity is probably not a defence to any offence of negligence. This is because the necessities of the situation are taken into account when determining the offence was committed in the first place: DPP v Harris  1 Cr App 170. Necessity will affect how a reasonable person would behave, so a necessary action is naturally non-negligent.
Necessity and Strict Liability
Necessity is usually available as a defence to strict liability offences: Santos v CPS  EWHC 550 (Admin). However, a correct interpretation of the statutory provision may lead to the conclusion that Parliament intended to exclude the defence: Cichon v DPP  Crim LR 918.