Since the doctrine of primacy can be applied before national courts, it also applies to labour law enforcement rules, including remedies and procedures. The CJEU has developed specific techniques and principles that can be used to enforce EU labour law before national courts. Where EU enforcement requirements conflict with national procedures and remedies, they prevail and must be applied by national courts that prevail over national rules. Notable examples include the Court`s decision in Marshall v. Southampton and South West Area Health Authority (No 2), Case C-271/91 (1993), on the removal of limits on compensation for sex discrimination in UK legislation. The Third Amendment to the Irish Constitution expressly provided for the primacy of Union law in Ireland by providing that no other provision of the Irish Constitution could invalidate laws enacted where membership of the European Communities so required. In Crotty v. As Taoiseach, the Irish Supreme Court held that Ireland`s ratification of the Single European Act was not required by accession to the European Communities and was therefore subject to judicial review. On 31 January 2020 at 23:00 GMT (00:00 CET in Brussels), the United Kingdom became the first member state to officially leave the European Union after 47 years of membership. This happened under the terms of the Brexit withdrawal agreement.
At the same time, the European Communities Act 1972 (ECA 1972), the law that introduced EU law (Community law as amended in 1972) into UK domestic law, was repealed by the European Union Withdrawal Act 2018, although the effects of the 1972 Act were preserved by the provisions of the European Union Act 2020 (Agreement of withdrawal) so that Union law can continue to produce legal effects within the EU. United Kingdom until the end of the transposition period ending on 31 December 2020. As the transposition deadline has expired, EU law no longer applies to the UK. However, the principle of the primacy of EU law applies to the interpretation of EU law that has been maintained.  Although the primacy of Community law over the national law of the Member States within the European Community has not always been enshrined in the EEC Treaty, it was firmly established in the case-law of the Court of Justice long before the United Kingdom`s accession to the Community. Therefore, any restriction on its sovereignty accepted by Parliament when it passed the European Communities Act 1972 was entirely voluntary. Under the provisions of the 1972 Act, it has always been clear that it is for a court in the United Kingdom, when giving a final judgment, to suspend any provision of national law contrary to a directly enforceable provision of Community law. On 14 March 2006, the Constitutional Court of Lithuania ruled in case No. 17/02-24/02-06/03-22/04, § 9.4 in Chapter III, that EU law takes precedence over ordinary acts of the Lithuanian Parliament, but not over the Lithuanian Constitution. If the Constitutional Court finds that EU law is unconstitutional, the old law loses its direct effect and remains inapplicable.  ⇒ The primacy of the EU over national law is possible because the 1972 Parliament adopted legislation to implement the supranational primacy of EU law (see section 2 of the European Communities Act 1972).
The legal doctrine of the primacy of EU law means that EU labour law takes precedence over national labour law. The creation of a new legal order of Union law and its primacy means that the Union institutions can adopt rules on employment and industrial relations, even if some Member States oppose those rules and vote against them in those EU institutions, provided that a majority voting procedure applies to that specific area. When these rules are adopted, they must be applied before national courts, even if they derogate from rules adopted by national legislative bodies. Over time, national constitutional courts have accepted the principles of the primacy of EU law confirmed by the Court of Justice of the European Union, while providing for a limitation of the fundamental principles of each national constitution. Like many other countries with a civil legal tradition, the French judicial system is divided into ordinary courts and administrative courts. The ordinary courts accepted the primacy of EU law in 1975, but the administrative courts did not accept this doctrine until 1990. The highest administrative court, the Council of State, had ruled that, since the administrative courts were not competent to review the legislation adopted by the French Parliament, they could not find the incompatibility of national legislation with Union law or give it authority of primacy over contrary national legislation. This contrasted with the ordinary supreme court, the Court of Cassation; in the case Administration des Douanes/Société “Cafés Jacques Vabre” and SARL Wiegel et Cie, it held that EU law should prevail over national law, in accordance with the requirements of Article 55 of the French Constitution, which gives primacy to the ratified international treaty over national law. The administrative courts finally changed their position in the Raoul Georges Nicolo case by deciding to follow the reasoning of the Court of Cassation. Depending on the constitutional traditions of the Member States, different solutions have been developed to resolve the problems of incompatibility between national and Union law. EU law is recognised as taking precedence over member state law, but not all member states share the CJEU`s analysis of why EU law takes precedence over national law in case of conflict. This predominance is well known in some areas, such as gender discrimination, where the domestic law of many Member States has been shaped by judgments of the European Court of Justice and has been repeatedly overridden in cases of conflict.
Perhaps the best-known example of the impact of these judgments on labour law is G. Defrenne v Sabena, Case 43/75, 1976,  ECR 455, in which the Court of Justice of the European Communities held that `the principle of equal pay for men and women enshrined in Article 119 EC [now Article 157 TFEU] may be relied on before the national courts. The task of those courts is to ensure the protection of the rights conferred on individuals by that provision, in particular in the case of forms of discrimination originating in legislation … Through the doctrine of primacy, EU law has promoted the Europeanisation of employment and industrial relations by ensuring that EU law applies in many areas of EU competence, from gender equality to worker representation.