The High Court in R(Miller) v. Secretary of State for Exiting the European Union has held that the UK government cannot trigger Article 50 of the Treaty on the European Union (TEU) without parliamentary approval, in the most important constitutional decision in recent years. The case has since been considered again by the Supreme Court.

Legal background – Brexit and Article 50

Following the Brexit referendum vote on 24 June 2016, there has been controversy and uncertainty about the process for leaving the EU. Article 50 TEU sets out the procedure by which a member state can leave the EU. The member state must notify the European Council of its intention to leave, and this is then followed by a two-year negotiation period for its exit.

The legal issue in question in this case was whether the government can trigger Article 50 without obtaining parliamentary authorisation, or whether it can instead rely on its royal prerogative powers without the need for such authorisation. The Claimants, Gina Miller and others, in this case argued for the former, and the Defendant, the government, argued for the latter.

The government's case: the royal prerogative

The government argued it can lawfully exercise its royal prerogative powers and leave the EU without the need to consult parliament. The royal prerogative powers are special powers that the Crown (or ministers acting on behalf of the Crown) has which mean that it does not need to consult with Parliament. Often, the conduct of foreign affairs and the making or breaking of treaties involve the use of these special powers. Further, nothing in EU law domestically limits their use.

The government argued that in exercising its powers, it is acting in accordance with the will of the majority of the population, as evidenced in the referendum result. The Referendum Act 2015 is silent on the issue of whether legislation needed to be passed to give notice under Article 50, but the government argued that it could be implied that it could act using its prerogative powers.

The Claimants' case: parliamentary approval

The Claimants based their case on Article 50(1), which requires that when a member state leaves the EU it must do so in accordance with its constitutional requirements. The Claimants contend that it is a constitutional requirement under the European Communities Act 1972 (ECA 1972) that the UK must obtain parliamentary authorisation in the form of an Act of Parliament.

The Claimants also raised the issue of timing. Both parties agree that triggering Article 50 is irreversible; and that it will be too late for Parliament to vote once the exit terms have already been negotiated with the EU.

The High Court decision

The Court agreed with the Claimants and held that the royal prerogative powers do not give the government authority to trigger Article 50 without parliamentary approval. The Court held that there was a conflict of constitutional principles in this case; but that the supremacy of Parliament is constitutionally higher than the royal prerogative powers. Further, the ECA 1972 had a fundamental impact on domestic law, to the extent that it cannot have been Parliament's intention for the rights it created to have been subject to changes via the royal prerogative powers.

Finally, the Court did not uphold the government's argument that the referendum result gave it an implied mandate to exercise its royal prerogative powers. The Referendum Act 2015 only made provision for an "advisory referendum", i.e. that the vote is powerfully influential but not legally binding on the government.

The High Court also made clear that its ruling was a legal, not a political one, and it did not reflect upon the merits of leaving the EU.

What next? Supreme Court appeal

The government has appealed the High Court decision and its case has been heard by the Supreme Court, with judgment expected in January 2017.

Its grounds for appeal are broadly the same as its High Court case: (1) the referendum legislation had been passed with a clear expectation that the government would implement the result; (2) the use of royal prerogative powers to trigger Article 50 would be a "classic exercise" of the powers; and (3) the ECA 1972 is a "conduit" or vehicle by which rights came into UK law but these can be amended or removed through use of the royal prerogative.

This is a divisive and high profile case which we will be monitoring as it unfolds.