Although not specifically stated within the legislation but due to the principle of Community Law (subsequently European Union Law) having primacy over the domestic national laws of the member states which was first established though the Costa v ENEL European Court of Justice ruling in 1964 as a consequence it also became binding on all legislation passed by the UK Parliament (and also upon the UK's devolved administrations—the Northern Ireland Assembly, Scottish Parliament and the Welsh Parliament (Senedd Cymru) although none of these institutions existed at the time of the passing of the Act).

On 13 July 2017, the then Brexit Secretary, David Davis, introduced what became the European Union (Withdrawal) Act to Parliament, which made provision for repealing the 1972 Act on "exit day", which was when enacted defined as 29 March 2019 at 11 p.m.(London time, GMT), but later postponed by EU decision first to either 22 May 2019 or 12 April 2019, later to 31 October 2019, and then again to 31 January 2020.

The repeal of these last remaining provisions ended the automatic incorporation into UK law of all future EU laws (with all previous EU laws being retained and transferred into UK law under the European Union (Withdrawal) Act 2018), and most future judgments of the ECJ as well as the regulations of the European Union Customs Union, the European Single Market, the Common Agricultural Policy and the Common Fisheries Policy, after 48 years on the statute book (with the exception to Northern Ireland under the terms of the Northern Ireland protocol), bringing to an end decades of political debate and discussions about the constitutional significance of the Act and its effect on the principle of Parliamentary sovereignty.

Almost immediately the British government regretted its decision, and in 1961, along with Denmark, Ireland and Norway, the UK applied to join the three Communities.

However, President Charles de Gaulle saw British membership as a Trojan Horse for US influence, and vetoed it; all four applications were suspended.

In October 1971, after a lengthy six day Commons debate on a white paper motion on the principle of accession, MPs voted 356–244 in favour of joining the EC.

Only three days after the signing of the Treaty, a European Communities Bill of just 12 clauses, was presented to the House of Commons by Geoffrey Rippon.

Denmark and Ireland also joined the Communities on the same day, 1 January 1973, as the UK; the Norwegian people had rejected membership in a referendum in 1972.

The European Communities Bill was introduced the House of Commons for its first reading by Geoffrey Rippon, Chancellor of the Duchy of Lancaster on 26 January 1972.

[6] The primacy of EU law in the United Kingdom during the time of its membership was derived from the European Communities Act in Section 2 (4).

This is seen by the legal scholar William Wade as a departure from the English constitutional doctrine of Westminster parliamentary sovereignty as it was and had been traditionally understood.

[9] The United Kingdom voted for withdrawal from the European Union in the referendum held on 23 June 2016, and afterwards there was speculation that the act would be either repealed or amended.

[14] This was achieved when the European Union (Withdrawal Agreement) Act 2020 was passed in January 2020 which "saved" the effect of the ECA until the end of the implementation period which was scheduled for 31 December 2020 before it was automatically repealed.