Direct effect is a basic principle
In Developing The Concept Of Direct Effect To What Extent, Has The European Court Of Justice Shown Itself To Be An “Activist” Court?
Direct effect is a basic principle of Community law. It was established by the Court of Justice of the European Communities in the Van Gend en Loos judgment. Direct effect confers rights on those, which they can invoke before the national, and Community courts.
This principle promotes Community law becoming part of national law and strengthens its effectiveness. In addition, it safeguards the rights of individuals in that they can invoke a Community provision, irrespective of whether a national text exists or not.
Although Community law is part of our legal system and is therefore directly applicable in the UK, not all Community law is directly effective, that is to say, capable of judicial enforcement. The Court of Justice has ruled in a number of cases that for a Treaty Article, Regulation or Decision to be directly effective, it must be sufficiently clear and unconditional for reliance to be placed on it, and there must be no scope for the exercise of Member State discretion in implementing it. Van Gend en Loos was the case, which first established the concept of direct effect of Community Law relating to Treaty Articles. A Dutch glue importer was required to pay bigger customs duty charges on imports by a Dutch law, which was implemented after the Treaty of Rome of 1958, had come into operation. Article 25 (ex 12) of the EC Treaty provided:
“Member States shall refrain from introducing between themselves any new duties on imports…and from increasing those which they already apply…”
The European Court of Justice decided that there were two varieties of direct effect: vertical direct effect and horizontal direct effect, the difference drawn being based on against whom the right is to be imposed.
Van Duyn v Home Office  ECJ was the first case to be referred to the European Court of Justice by an English court. Miss Van Duyn was refused leave to enter the UK on the grounds of her undesirability. She was a Dutch national and a practising Scientologist.
Miss Van Duyn attempted to rely on article 48 of the Treaty, and Art 3 of Directive 64/221, which allowed free movement of workers in the EU.
Since the directive laid down an responsibility, which was not subject to any exclusion or condition, and by its nature did not require involvement on the part of the Community or Member State, it was to be regarded as directly effective conferring enforceable individual rights, which national courts must protect. However, a member state could refuse leave to enter its territory on the ground of association with an organisation whose activities were deemed to be contrary to the public good. It was irrelevant that the organisation is not unlawful and that nationals of member state are permitted to work for it. Miss Van Duyn was refused entry.
In Grad v Finanzamt Traunstein in which a decision addressed to the German government was held to be directly effective. However, restrictions were placed on the doctrine of direct effect. These have to ensure that national courts do not have difficulty in applying Community law and that the measure or provision in question must be unconditional and sufficiently precise, along with leaving no discretion to the Member State as to how it should be implemented are not strictly applied. In Van Duyn, the ECJ stated that if there was doubt as to how a particular provision of Community law should be construed, the national court could refer the question to the ECJ under the preliminary reference procedure.
In Defrenne v SABENA (No 2)  ECJ it was held that Treaty provisions, in this case the provision for equal pay for equal work in Article 119, could be invoked against private employers. However, while the concept of horizontal direct effect added further protection to individuals' Community law rights, at the same time it appeared to breach the principle of legal certainty.
Marshall v Southampton & SW Hampshire AHA  ECJ case concerned the effect of Directive 76/207 on Equal Treatment of Men and Women in Employment. The ECJ held that the Directive had direct effects, and the legal basi was stated to be that the State could not rely in court on its own failure to meet its obligations; given this legal basis, the Court then continued to hold that Directives therefore had vertical direct effect and not horizontal direct effect.
On the whole, it would appear that the Court has acted less out of regard for strict legal principles and is more of a supporting body. Nevertheless, the Court was certainly persuaded by the rebellion from the French Conseil d'Etat and the German Bundesfinanzhof. Anderson, writing at the time, commented that: “The present concern of the Court is to consolidate the advances of the 1970s rather than face the legal complexities and political risks of attempting to extend the doctrine further”
Van Gend en Loos v Nederlanse Administratie der Belastingen (Case 26/62)  ECR 1
Gleaned from Elspeth Deards and Sylvia Hargreaves: European Union Law, 1st Edition, Oxford University Press, 2004, page 79, an extract from the EEC Treaty of Rome 1958, Article 25 (ex 12)
Van Duyn v Home Office 41/74  ECR 1337
Communities, 3rd edition, edited and further revised by Laurance W fifth edition, Kluwer Law International, page 530
Grad v Finanzamt Traunstein 9/70  ECR 825
Defrenne v SABENA (No 2)  ECJ 43/75  ECR 455
Marshall v Southampton & SW Hampshire AHA  ECJ 152/84  ECR 723;  1 CMLR 688
Anderson D, Boston College International and Comparative Law Review (1988) XI 91, 100, cited in Weatherill S, ‘Cases and Materials on EC Law', 2nd edition, 1994, Blackstone, at p.80