The UK left the EU on 31 January 2020 at 11pm GMT with a ratified Withdrawal Agreement. The changes brought about by Brexit have been implemented predominantly through the EU (Withdrawal) Act 2018 and the EU (Withdrawal Agreement) Act 2020. This briefing paper explains the constitutional and legal implications of those two Acts, taken together, and what it means for the next part of the Brexit process.

The UK ceased to be a Member State of the European Union with the coming into force of the Withdrawal Agreement (WA) on 31 January 2020 at 11pm GMT. That treaty governs the current relationship between the UK and the EU regarding several matters, while negotiations on more permanent arrangements about trade, security and political cooperation are undertaken.

This change in relationship with the European Union has had major domestic and constitutional consequences for the United Kingdom.

This briefing paper explains the changes in domestic law brought about by this new settlement, and their constitutional significance. It focuses especially on the relationship between two Acts of the UK Parliament: the European Union (Withdrawal) Act 2018 (EUWA) and the European Union (Withdrawal Agreement) Act 2020 (WAA). It explains:

  • how the “transition or implementation period” (hereafter “transition” or “the transition period”) is being implemented;
  • what powers the UK Government (and devolved institutions) have for the purposes of implementing the Withdrawal Agreement;
  • what powers the UK Government (and devolved institutions) have regarding retained EU law after the transition period has ended;
  • what role Parliament has in scrutinising the implementation of the Withdrawal Agreement and subsequent stages of UK-EU negotiations;
  • what continuing role the Court of Justice of the European Union (CJEU) will have on UK law in light of the terms of EU exit; and
  • what withdrawal from the EU means in a broader sense for the fundamental principles of the UK constitution, such as the sovereignty or legislative supremacy of Parliament.

The paper also addresses constitutional issues that have resulted from the process of legislating for Brexit itself. Leaving the EU, and legislating for it, have had knock-on effects for:

  • devolution, devolved competence and the legislative consent convention;
  • Parliament’s role in scrutinising and approving constitutionally significant treaty change; and
  • the ability of Parliament to scrutinise and constrain the delegation of decision-making from itself to Government Ministers and executive agencies.