# Sources of EU law

> **Key takeaway:** Treaties + general principles (primary). Regulations/directives/decisions (secondary). CJEU case law interprets. Directives need implementation unless direct effect. UK today: mainly assimilated law / TCA — see REUL note.

- **Jurisdiction:** England & Wales
- **Practice area:** Public Law
- **Last reviewed:** 2026-08-04
- **Interactive page:** https://kttclegal.info/library/notes/PublicLaw/eu-sources-of-law
- **Keywords:** sources of EU law, regulation, directive, TEU, TFEU, Van Gend en Loos, acquis, secondary legislation

## What is this about?

During UK membership, EU law derived from treaties, secondary legislation (regulations, directives, decisions), general principles, and Court of Justice case law. Post-Brexit, those sources matter historically and where assimilated law or the Trade and Cooperation Agreement still tracks EU concepts. This note maps the classic source hierarchy for reference.

## What is the core rule?

Primary EU law comprises the Treaties (TEU/TFEU and predecessors) and general principles/fundamental rights as recognised in the EU legal order. Secondary law includes regulations (generally directly applicable), directives (binding as to result; require implementation), and decisions. International agreements concluded by the EU and CJEU jurisprudence interpreting EU law formed part of the enforceable acquis for Member States. Hierarchy: treaties and general principles constrain secondary acts; secondary acts must have a legal base.

## What are the elements or test?

1. Is the instrument primary (treaty) or secondary (regulation/directive/decision)?
2. Does it require domestic implementation (typical directive) or direct application (typical regulation)?
3. What legal base and legislative procedure applied?
4. For UK practice today: is the rule assimilated domestic law, TCA-related, or purely historical?
5. Role of CJEU case law under EUWA s.6 as amended

## Which authorities matter?

- **Treaty on European Union; Treaty on the Functioning of the European Union (as applicable pre-exit)** — Primary treaty framework of the modern EU legal order.
- **Van Gend en Loos (Case 26/62) [1963] ECR 1** — Foundational CJEU recognition of a new legal order and direct effect of treaty provisions meeting the test.
- **European Union (Withdrawal) Act 2018 (as amended); Retained EU Law (Revocation and Reform) Act 2023** — Domestic status of former EU law after exit — see retained EU law corpus note.

## How does this apply in practice?

For current UK disputes, start with domestic statute/regulations and the retained/assimilated law note. Use this note to decode EU parentage and historical authorities. Do not assume regulations still 'apply' as EU instruments in the UK.

## What are common pitfalls?

- Citing directives as if they automatically create rights without implementation/direct effect analysis
- Treating pre-exit EU source hierarchy as unchanged post-2023 reforms
- Ignoring legal base challenges that were central to EU validity review

## When would a practitioner use this?

Interpreting assimilated law with EU parentage; academic/historical analysis; cross-border matters still engaging EU instruments abroad.

## Quick reference

Treaties + general principles (primary). Regulations/directives/decisions (secondary). CJEU case law interprets. Directives need implementation unless direct effect. UK today: mainly assimilated law / TCA — see REUL note.

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*Reference material from [KTTC Legal](https://kttclegal.info/), not legal advice. Work product supports instructing solicitors and barristers under their supervision. England & Wales.*