# UK EU membership, withdrawal, and the Trade and Cooperation Agreement

> **Key takeaway:** In via ECA 1972; out via Art 50 + WA (EUWAA 2020) + EUWA 2018. TCA = FTA-style cooperation, not membership. Assimilated law continues many rules. Miller (No 1) on triggering exit. Check NI/Windsor separately.

- **Jurisdiction:** England & Wales
- **Practice area:** Public Law
- **Last reviewed:** 2026-08-04
- **Interactive page:** https://kttclegal.info/library/notes/PublicLaw/uk-eu-membership-and-tca
- **Keywords:** Brexit, Article 50, Withdrawal Agreement, Trade and Cooperation Agreement, ECA 1972, Miller, EUWAA 2020, Windsor Framework

## What is this about?

The UK joined the EEC in 1973, gave EU law domestic effect through the European Communities Act 1972, and left the EU under Article 50 following the 2016 referendum. The Withdrawal Agreement and the UK–EU Trade and Cooperation Agreement (TCA) now structure residual obligations and trade cooperation alongside assimilated EU-derived law.

## What is the core rule?

Membership: ECA 1972 was the domestic conduit for EU law supremacy/direct effect while a Member State (Factortame era). Withdrawal: Article 50 TEU notification and the Withdrawal Agreement (implemented by EUWAA 2020) covered separation issues including citizens' rights and the Northern Ireland arrangements (as amended). The TCA (implemented domestically as needed) provides a free-trade-agreement-style framework for goods/services cooperation, level-playing-field commitments, and governance without single-market free movement. Retained/assimilated law preserves much of the domestic acquis subject to reform (see REUL note).

## What are the elements or test?

1. Is the issue historic membership law, WA citizens' rights, NI Protocol/Windsor Framework, TCA trade, or pure assimilated law?
2. Which implementing UK statute applies?
3. Any continuing CJEU role under the WA in limited fields?
4. Domestic enforceability of TCA provisions?

## Which authorities matter?

- **European Communities Act 1972 (repealed subject to savings)** — Historic domestic foundation of EU law effect in the UK.
- **European Union (Withdrawal) Act 2018; European Union (Withdrawal Agreement) Act 2020** — Exit-day architecture and Withdrawal Agreement implementation.
- **R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5** — Supreme Court on the need for primary legislation to trigger Article 50 in domestic law.
- **EU–UK Trade and Cooperation Agreement (and UK implementing legislation)** — Post-transition trade and cooperation framework — check current consolidated text.

## How does this apply in practice?

Northern Ireland goods rules are specialised and politically fluid — verify current Windsor Framework instruments. Citizens' rights under the WA are a distinct enforceable regime. Do not conflate TCA with single-market membership.

## What are common pitfalls?

- Treating the TCA as recreating free movement of persons
- Ignoring WA citizens' rights when advising EU nationals
- Assuming all EU law vanished on exit day rather than being retained/assimilated

## When would a practitioner use this?

Brexit-related statutory interpretation; citizens' rights; trade compliance framing.

## Quick reference

In via ECA 1972; out via Art 50 + WA (EUWAA 2020) + EUWA 2018. TCA = FTA-style cooperation, not membership. Assimilated law continues many rules. Miller (No 1) on triggering exit. Check NI/Windsor separately.

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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.*