The non-written sources of European law: supplementary law

Article 19 Treaty on European Union

Article 19 of the Treaty on European Union (TEU) describes the role and composition of the Court of Justice of the European Union (CJEU).
It forms the legal basis of supplementary law by designating the CJEU as the ultimate court to ensure that EU law is correctly interpreted and applied, whether on actions brought directly to it or on requests for preliminary rulings* (for more information, see the summary on Preliminary ruling proceedings).

What does supplementary law mean?

Supplementary law designates the unwritten sources of EU law as opposed to the primary and secondary sources.
These sources are essentially used by the CJEU as rules of law in cases where the primary and secondary legislation do not settle the issue.

What are the supplementary sources of law?

The supplementary sources include the case-law* of the CJEU, international law* and the general principles of law. These 3 sources overlap.

The case-law of the CJEU
In interpreting EU law, the CJEU often needs to be creative in order to bridge the gaps left by primary law and/or secondary law.

Its judgments provide an official and legally binding interpretation of EU law and, as such, are referred to as a source of law (including by the CJEU itself).

International law

International law (including international agreements concluded by the EU and EU countries) is used by the CJEU when developing its case-law, in so far as its rules apply to the EU and they are not incompatible with the specific structure of the EU.

Besides the legal commitments of the EU and EU countries, international law is also a source of inspiration for the CJEU when drawing up general principles of law.
The general principles of law

Developed by the case-law of the CJEU, general principles have allowed the Court to implement rules in different domains of which the treaties make no mention.

General principles of EU law may be common to all the national legal systems of the EU countries and compatible with EU objectives, or specific to the EU, even if inspired from principles enshrined in certain national legal systems only.

Fundamental rights

Fundamental rights (Article 6 of the Treaty on European Union (TEU)) are a special category of source of law:
partly primary — Article 6(1) TEU recognises the Charter of Fundamental Rights of the European Union as having the same legal value as the Treaties;
partly supplementary — Article 6(3) TEU explicitly recognises as general principles of EU law ‘fundamental rights, as guaranteed by the Convention for the Protection of Human Rights and Fundamental Freedoms and as they result from the constitutional traditions common to the Member States’.


Preliminary ruling: a ruling by the Court of Justice of the European Union in reply to a question from a national court on the interpretation or validity of EU law, thereby contributing to the uniform application of that law.
Case-law: the law as established by the outcome of former cases.
International law: the set of rules that governs the legal relations between or among states or nations.


Consolidated version of the Treaty on European Union — Title III — Provisions on the institutions — Article 19 (OJ C 202, 7.6.2016, p. 27)

Consolidated version of the Treaty on the Functioning of the European Union — Part One — Principles — Title I — Categories and areas of Union competence — Article 6 (OJ C 202, 7.6.2016, pp. 52-53)

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