The provisions of the Treaties are the primary source of EU law. The doctrine of supremacy of EU law means that Treaty provisions take precedence over domestic laws (Flaminio Costa v. ENEL, Case 6/64). Equally, treaty provisions containing rules on employment and industrial relations take precedence over domestic labour law rules. The European Court of Justice may attribute to some Treaty provisions the legal quality of ‘direct effect’: individuals may directly cite them before national courts (NV Algemene Transporten Expeditie Onderneming van Gend en Loos v. Nederlandse Administratie der Belastingen, Case 26/62). Treaty provisions can be used, therefore, to make claims before domestic courts and override domestic law. For example, in Defrenne v. Sabena, Case 43/75, the European Court of Justice held: ‘The principle that women and men should receive equal pay, which is laid down by Article [141 EC, now Article 157 TFEU], may be relied on before the national courts’. Claims based on treaty provisions with direct effect may be made against the state and emanations of the state (‘vertical direct effect’), and against private individuals or corporations (‘horizontal direct effect’).
However, there was little law on employment and industrial relations in the articles of the original Treaties of Rome. Most EU law on employment and industrial relations is found in other EU legal measures, in particular directives. Following the amendments to the Treaties of Rome by the Single European Act, the Treaty of Maastricht (Treaty on European Union), the Treaty of Amsterdam, the Treaty of Lisbon, the Treaty provisions in the area of employment and industrial relations now have a more constitutional function. They determine the competences of the EU in the area of employment and industrial relations: for example, the Employment Title (Articles 145-150 TFEU) and the ‘Social Chapter’, (Articles 151-161 TFEU). Treaty provisions also determine how these competences may be exercised: for example, through legislation (Article 153 TFEU), through the European social dialogue (Articles 154-155 TFEU) or through the ‘open method of coordination’ (Article 148 TFEU).