Principle of Legality in Criminal Law

Nullum crimen sine lege means “no crime without law”: criminal punishment may be imposed only for conduct that a valid law had already made an offence. The broader principle of legality also requires the offence and its punishment to be defined clearly enough to guide individuals and restrain 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.

Constitutional protection in India

Article 20(1) protects every person from conviction for an act that was not an offence under the law in force when it was committed. It also prohibits imposition of a greater penalty than the penalty applicable at that time.

• The protection concerns retrospective creation of substantive criminal liability and retrospective enhancement of punishment.
• It applies to criminal conviction and punishment, rather than ordinary civil liability.

Connection with separation of powers

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