UK High Court rules against MPs withdrawing from EU without Act of Parliament

R (on the application of Miller and another) v Secretary of State for Exiting the European Union [2017] UKSC 5 (24 January 2017)


The majority in the High Court of Justice has ruled that ministers of the United Kingdom cannot employ prerogative powers to withdraw from the European Union (EU) without an Act of Parliament authorising them to do so.

Their decision relied on the notion that only an Act of Parliament can significantly alter the UK's constitutional arrangements. As the European Communities Act 1972 (the 1972 Act) established that European Union law was a significant source of UK domestic law, the majority ruled that this legal source could not be removed by ministerial decision alone.


On 22 January 1972, United Kingdom ministers signed a Treaty of Accession making the UK a member of the European Economic Community (EEC) as of 1 January 1973. Following significant transformations over the past 40 years, the EEC is now known as the European Union (as of 2009) and has greater competences.

The 1972 Act was passed by the UK Parliament in October 1972 and enabled UK ministers to ratify the 1972 Accession Treaty. This legislation also provided that "all such rights, powers, liabilities, obligations and restrictions from time to time created or arising by or under the (EU) Treaties" would be "recognised and available" in UK domestic law.

The list of EU Treaties, as identified in section 1(2) of the 1972 Act and which gave rise to these rights and restrictions, could be modified. Since 1975, a number of Treaties have been added to the list of Treaties in section 1(2) of the 1972 Act including Titles II, III and IV of the Maastricht Treaty on the European Union of 7 February 1992 (TEU).

Article 50 was introduced into the TEU in the Lisbon Treaty and stated:

  1. Any member state may decide to withdraw from the Union in accordance with its own constitutional requirements.
  2. A member state which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that state, setting out the arrangements for its withdrawal.
  3. The Treaties shall cease to apply to the state in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European council, the Union shall negotiate and conclude an agreement with that state, setting out the arrangements for its withdrawal.

On 23 June 2016, a referendum was held in the UK which indicated that the majority was in favour of leaving the EU.

The main constitutional issue for the court to decide was whether UK ministers alone have the power to give notice under Article 50(2) of the TEU and withdraw from the EU or whether they must be authorised to do so by an Act of Parliament. The court also considered the role of the statutorily devolved legislatures of Scotland, Wales and Northern Ireland and the extent to which consultation with these legislatures was required in the circumstances.


(a) UK constitutional issue

The main constitutional issue in this case was brought by Gina Miller and Deir dos Santos (the Applicants) against the Secretary of State for Exiting the European Union (the Respondent) in the Divisional Court of England. The Applicant successfully argued that giving Notice as per Article 50(2) of the TEU would have the effect of altering UK domestic law and would, therefore, be unconstitutional.

However, the Secretary of State appealed the judgment in the High Court of Justice, arguing that the well-established prerogative power for ministers to enter into and withdraw from treaties should extend to the treaty governing EU membership.

The two central constitutional principles as accepted by the court in this case were that:

  1. Ministers have a well-established prerogative power to enter into and withdraw from treaties without recourse to Parliament; and
  2. Ministers do not have the power to change UK domestic law without an Act of Parliament.  

Therefore, the court had to consider whether it would be unconstitutional for UK ministers to give Notice under Article 50(2) of the TEU on the grounds that it altered domestic law.

Majority decision

The majority acknowledged that, while prerogative powers cannot change domestic law, they may still be validly exercised if they have legal consequences. However, they emphasised that the prerogative power to make and unmake treaties relies on the fact that (1) treaties have effect in international law (which operates on a different "sphere" to UK domestic law) and (2) treaties are binding on the UK only in international law and are not part of UK domestic law, so do not give rise to domestic legal rights and obligations.

In these circumstances, the 1972 Act was unprecedented. The majority argued that it made EU law a source of UK domestic law such that rights, duties and rules derived from EU law now applied in the UK. However, the 1972 Act went further to create a new constitutional process for making law in the UK. The 1972 Act was not the source of the law but rather, gave effect to a new source of UK domestic law, EU law itself.

Therefore, the majority found that withdrawing from the EU would significantly change UK domestic law and UK citizens would lose a number of their rights (eg. derived from EU employment law, EU competition law).

The Respondent sought to argue that the 1972 Act itself had sanctioned the loss of rights from "time to time" since the EU Treaties were subject to change and the Act made allowances for this. However, the majority rejected this argument as they saw a vital difference between changes in UK domestic law resulting from variations in the content of EU Treaties and changes resulting from withdrawal by the UK from the EU altogether. The majority argued that the latter involved a "fundamental change in the constitutional arrangements of the UK" given that EU law would cease to be a source of UK domestic law.

The majority further noted that the European Union Referendum Act 2015 had no effect on its decision as it had not expressly provided for a change in the legislation in response to the electorate's vote.

Therefore, they argued that it would be unconstitutional for UK ministers to give Notice under Article 50(2) of the TEU without Parliamentary legislation.

Dissenting opinions

Three judges argued that the constitution was not impinged if UK ministers were to give Notice to withdraw from the EU under Article 50(2) of the TEU.

Lord Reed, with whom Lord Hughes agreed, argued that the Crown was not altering UK domestic law in withdrawing from the EU Treaties. He analysed the wording of s 2(1) of the 1972 Act to suggest that the Parliament had envisaged circumstances where the "rights" as established by EU Treaties may be "added to, altered or revoked" without a further Act. He did not believe there to be a distinction between a change in these rights from a variation of an EU treaty and UK withdrawal from the EU treaties.

Lord Reed further argued that the 1972 Act was the source of law giving effect to the EU obligations and therefore, UK constitutional arrangements would not be affected if EU law no longer applied to the UK.

Lord Carnwath acknowledged the fundamental principle as agreed by all judges that prerogative powers cannot change the law or alter/remove existing rights. However, he argued that, if the Crown were to give Notice under Article 50(2) of the TEU, this would only commence a process which may lead to future changes. Of itself, the minister's Notice would not change the law or alter/remove any existing rights as an "intense period of negotiations" would follow to facilitate the legal effect of the decision.

(b) Devolution issues

All judges agreed that devolution issues did not prevent the UK Parliament from legislating (or, in the case of the dissenting judges, from UK ministers giving Notice) such that the UK could withdraw from the EU.

The main devolution issue discussed by the court was the effect of the Sewel Convention. This convention applies to the UK Parliament when legislating with respect to the devolved legislatures- Scotland, Wales and Northern Ireland. The UK Parliament seeks to cooperate with these devolved legislatures but reserves the right to legislate on matters affecting them without their consent.

However, the convention is political in nature so the court is unable to give legal rulings on its operation and scope. Therefore, consent from the devolved legislatures is not required by law before an Act of Parliament empowers UK ministers to withdraw from the EU.  


This important decision confirms the limits of the powers of the ‘royal prerogative’ of Ministers to act on behalf of the crown. It also sets a critical precedent about the precise scope of executive power in relation to withdrawal from international treaties.

On 16 March 2017, the two line European Union (Notification of Withdrawal) Act 2017 (UK) came into effect, granting the Prime Minister the power to notify withdrawal from the EU. On 29 March 2017, UK Prime Minister Theresa May invoked Article 50 of the Lisbon Treaty - the first time it has ever been used - to notify the EU of the UK’s intention to withdraw. The UK now has until 29 March 2019 to negotiate its withdrawal from the EU.

As of May 2017, the UK Government has released a white paper on the Great Repeal Bill on legislating for the UK’s withdrawal from the EU by repealing the European Communities Act 1972 (UK) while retaining the majority of EU laws under UK laws. This process will have significant impacts on both UK and EU citizens in the UK, particularly in relation to fundamental rights and avenues for review under the European Convention of Human Rights.

The full text of the decision is available here.

Ray Mainsbridge is a Senior Consultant and Anna Gruen is a Graduate at Ashurst.