What is the principle of legality?

The principle of legality is one of the fundamental pillars of the rule of law and refers to the fact that the public authority is subject to the law and can only act under the terms and conditions established by the law. In other words, the principle of legality establishes that any action by the public authority must be based on a pre-existing and valid rule of law.

This principle is essential to guarantee the protection of individual rights and freedoms, as well as to prevent the abuse of power by public authorities. In this sense, the principle of legality is a limit to power and a guarantee for citizens.

Manifestations of the principle of legality

The principle of legality has several manifestations and scopes, which are described below:

1. Respect for the Constitution and the laws: The principle of legality implies that all public authorities are obliged to respect and comply with the Constitution and the laws emanating from it. This means that no authority may act contrary to the provisions of the law.
2. Prohibition of arbitrariness: The principle of legality also implies that public authorities may not act arbitrarily, i.e. without a valid legal basis. This means that all actions of the public authorities must have a legal justification and be based on valid legal norms.
3. Prohibition of retroactivity of the law: The principle of legality establishes that laws may not have retroactive effects, i.e. they may not affect events that occurred prior to their entry into force. This guarantees legal certainty and the protection of acquired rights.
4. Guarantee of fundamental rights and freedoms: The principle of legality also implies that the public authorities must ensure that the fundamental rights and freedoms of individuals are respected and protected. This means that the public authorities are obliged to act always within the framework of the law and with respect for individual rights and freedoms.
5. Judicial review of legality: The principle of legality implies that the decisions and actions of the public authorities can be reviewed by the courts. This means that persons affected by a public authority's actions may appeal to the courts to defend their rights and that the courts may declare an action that is not in accordance with the law to be null and void.

How does the principle of legality apply?

Some of the ways in which the principle of legality is applied are:

• In the field of criminal law: The principle of legality is of great importance in criminal law, as it states that no one can be convicted of an offence that is not defined as such in a previous law. This means that judicial and police authorities must always act within the framework of the law and that any action that does not comply with the relevant legal rule may be considered illegal and therefore void.
• In the field of administrative law: The principle of legality also applies in the field of administrative law, as it establishes that all actions of administrative authorities must be based on a prior legal rule and that any action that does not conform to such a rule may be considered unlawful. This principle also establishes that administrative authorities must respect individual rights and freedoms, and that any action that violates them may be declared null and void.
• In the field of labour law: The principle of legality also applies in the field of labour law, as it states that all employment relationships must be based on a prior legal norm, such as an employment contract or a labour law. This means that any action that violates workers' rights or does not comply with labour standards may be considered illegal and therefore void.
• In the field of tax law: The principle of legality also applies in the field of tax law, as it states that all actions of the tax authorities must be based on a prior legal rule, such as a tax law or a tax regulation. This means that any action that violates taxpayers' rights or does not comply with tax rules may be considered illegal and therefore void.

What are the elements of the principle of legality?

The main elements of the principle of legality are:

1. The existence of a prior rule of law: The principle of legality requires that all actions of public authorities must be based on a prior rule of law, i.e. that they must be in accordance with the law. This means that authorities may not act in an arbitrary or discretionary manner, but must follow procedures established by law.
2. The requirement of legal reservation: The principle of legality also requires that certain matters be regulated exclusively by law and not by other sources of law, such as regulations or administrative decisions. This is known as the requirement of legal reservation and aims to ensure that decisions affecting individual rights and freedoms are taken only through established democratic procedures.
3. The prohibition of retroactivity of the law: The principle of legality prohibits retroactivity of the law, i.e. the law being applied with retroactive effect to events that occurred before its entry into force. This means that legal rules must apply prospectively and may not affect situations that have already occurred.
4. The guarantee of individual rights and freedoms: The principle of legality requires that the actions of public authorities respect individual rights and freedoms, and that any actions that violate them can be declared null and void. This means that authorities must always act in accordance with democratic principles and in protection of human rights.
5. The requirement of a restrictive interpretation of criminal law: In the field of criminal law, the principle of legality requires that criminal laws be interpreted restrictively, i.e. that they be applied only to those cases in which all the elements foreseen in the law are fulfilled. This means that judicial authorities may not extend the scope of a criminal law beyond what is provided for in the legal norm.
6. The requirement of a sufficient statement of reasons for administrative decisions: In the field of administrative law, the principle of legality requires that all administrative decisions be sufficiently reasoned, i.e. that the legal and factual basis of the decision be clearly and precisely explained. This enables citizens to understand decisions that affect their rights and to challenge them if they consider that they violate their rights.