Rule 101. No one may be accused or convicted of a criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time it was committed; nor may a heavier penalty be imposed than that which was applicable at the time the criminal offence was committed.
Volume II, Chapter 32, Section N.
State practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts.
The Third and Fourth Geneva Conventions provide that prisoners of war and civilians respectively may not be tried for acts that were not criminal offences, provided for by law, prior to the commission of those acts.
Additional Protocols I and II repeat the same principle and add that a heavier penalty may not be imposed than that applicable at the time the act was committed but that if, subsequent to the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit from this.
This principle of legality is also set forth in the Statute of the International Criminal Court.
The principle of legality is set forth in several military manuals and is part of most, if not all, national legal systems.
The principle of legality, including the prohibition on imposing a heavier penalty than that applicable at the time of the commission of the offence, is set forth in the International Covenant on Civil and Political Rights, the Convention on the Rights of the Child and the regional human rights conventions.
It is specifically listed as non-derogable in the International Covenant on Civil and Political Rights and the European and American Conventions on Human Rights,
while the Convention on the Rights of the Child and the African Charter on Human and Peoples’ Rights do not allow for the possibility of derogations. In addition, the International Covenant on Civil and Political Rights and the American Convention on Human Rights specify that if, subsequent to the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit from this.
The principle of legality is also contained in other international instruments.
The principle of legality has been interpreted by the European Court of Human Rights as embodying the principle that only the law can define a crime and prescribe a penalty and the principle that criminal law must not be extensively construed to an accused’s detriment, for instance by analogy. This requires that the offence be clearly defined in law, so that “the individual can know from the wording of the relevant provision and, if need be, with the assistance of the court’s interpretation of it, what acts and omissions will make him liable”.
The European Court of Human Rights has stated that the principle of legality allows courts to gradually clarify the rules of criminal liability through judicial interpretation from case to case, “provided that the resultant development is consistent with the essence of the offence and could reasonably be foreseen”.
The Inter-American Court of Human Rights has also stressed that the principle of legality requires that crimes be classified and described in “precise and unambiguous language that narrowly defines the punishable offence”.