There are four main legality principles;

1. Punishability requires a written act of parliament and cannot be based on custom. (No Punishment without Law)

2. Criminal prohibitions must determine the prohibited conduct, they must be definite (the principle of certainty)

3. Acts cannot be punished retroactively. (the principle of non retroactivity) The penalty and any ancillary measures shall be determined by the law which is in force at the time of the act. If the penalty is amended during the commission of the act, the law in force at the time the act is completed shall be applied. If the law in force at the time of the completion of the act is amended before judgment, the most lenient law shall be applied. A law intended to be in force only for a determinate time shall be continued to be applied to acts committed while it was in force even after it ceases to be in force, unless otherwise provided by law.

4. The statutory prohibition cannot be extended by analogy to conduct not covered by the ordinary meaning of the words used. (the principle of the interdiction of analogy).  The ECHR, with its Article 7, defines the principle as ‘No one shall be held guilty of any criminal offense on account of any act or omission which did not constitute a criminal offense under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offense was committed.’