In any rule-of-law-based criminal justice systems, it is of key importance that the state powers to investigate, prosecute and punish (ius puniendi) are not only granted by law, but also subject to the command and control of law. This control restraint is the very essence of the rule of law and the ‘Rechstaat’, as the nation-state has to guarantee both the liberty and security of citizens. This has not only consequences for the way in which substantive criminal law is enacted, following the maxim of Beccaria: ‘nullum crimen, nulla poena sine lege’, but also for the bridge between substantive criminal law and the empowerment of use of investigative and prosecutorial powers by judicial authorities, like a prosecutor. A suspect has to know for which offences he can be held criminally liable and what penalties could be applied (substantive legality). These offences and penalties must be laid down in clear, specific and definite terms (lex certa) and be foreseeable before the commission of the acts (lex praevia).
John A. E. Vervaele
(The Material Scope of Competence of the European Public Prosecutor’s Office: Lex uncerta and unpraevia? în The needed balances in EU Criminal Law: past, present and future, Hart Publishing, 2018, la p. 413)