"Universal suffrage means in principle that all human beings have the right to vote. This right may, however, and indeed should, be subject to certain conditions: d. Deprivation of the right to vote: i. provision may be made for depriving individuals of their right to vote, but only subject to the following cumulative conditions: ii. it must be provided for by law; iii. the proportionality principle must be observed; iv. the deprivation must be based on mental incapacity or a criminal conviction for a serious offence; v. furthermore, the withdrawal of political rights or finding of mental incapacity may only be imposed by express decision of a court of law. "
CoE (Venice Commission): Code of Good Practice on Referendums, para. 1.1.d

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Code of Good Practice on Referendums