Principle of Legality in Criminal Law

Nullum crimen sine lege means “no crime without law”: punishment may be imposed only for conduct that a valid, pre-existing law had made an offence. The broader principle of legality requires criminal offences and penalties to be prescribed clearly by lawful authority, thereby guiding individuals and restraining arbitrary state action.

Requirements of legality

• A pre-existing law, or lex praevia, must prohibit the conduct when it is committed; subsequent criminalisation cannot operate retrospectively.
• The offence must be stated with reasonable clarity and certainty, or lex certa, so that people can foresee what conduct attracts criminal liability.
• Criminal liability must rest on a law made by a competent lawmaking authority, not merely on morality, custom or an executive preference.
• A penal provision must not be extended through analogy to conduct that its language does not cover; courts may interpret the law but cannot create a new offence.

Article 20(1) and ex post facto laws

An ex post facto criminal law seeks to attach criminal liability or a harsher punishment to conduct already completed. Article 20(1) prevents such retrospective operation through two protections.

• A person cannot be convicted for an act that was not an offence under the law in force when the act was committed.
• A person cannot be subjected to a greater penalty than the penalty applicable when the offence was committed.
• The protection concerns retrospective substantive criminal liability and enhancement of punishment; it does not bar retrospective civil liability or procedural changes that do not create an offence or increase punishment.
• Article 20(1) does not prevent retrospective application of a law that reduces punishment.

Connection with separation of powers

The decision to criminalise conduct and prescribe punishment belongs to the legislature, acting within constitutional limits. The judiciary interprets and applies the law, but cannot cure a legislative omission by declaring previously non-criminal conduct to be an offence.