Two days later, the government responded by bringing to Parliament the European Union (Notification of Withdrawal) Act 2017 for first reading in the House of Commons on 26 January 2017.

[5] Following a referendum held on 23 June 2016, in which 51.9% of votes cast were in favour of leaving the EU, the UK government stated its intention to invoke Article 50 of the Treaty on European Union (the formal procedure for withdrawing) on 29 March 2017.

On the Monday following the referendum, three academics (Nick Barber, Tom Hickman and Jeff King) published a blog which argued that an Act of Parliament would be necessary before the Government could give notice to leave the EU.

David Davis, the Secretary of State for Exiting the European Union, argued that the possibility to trigger Article 50 was based on the royal prerogative and so any consultation of elected members of parliament was unnecessary.

The UK's constitutional requirements for the valid invocation of Article 50 was the entire basis of this litigation, even though this was undertaken without explicit reference to that phrase as in Art 50(1) in the judgments.

The first of the parties to lodge a complaint in the proceedings against the government's intention to trigger Article 50 without a parliamentary vote was Deir Dos Santos, who launched his action four days after the referendum of 23 June.

[19] The law firm Mishcon de Reya announced that it had been retained by a group of clients to challenge the constitutionality of invoking Article 50 without Parliament debating it.

[20][21] In the proceedings, all parties accepted that withdrawal from the European Union would have profound consequences in terms of changing domestic law in each of the jurisdictions of the United Kingdom.

[26] At the full hearing in October, before three judges sitting as a divisional court (the Lord Chief Justice, the Master of the Rolls and Lord Justice Sales), it was argued for the lead claimant (Miller) that notification under Article 50 would commit the UK to the removal of rights existing under the European Communities Act 1972 and later ratification acts, and that it is not open to the government, without Parliament's approval, to use the prerogative power to take action affecting rights which Parliament had recognised in that way.

[27] An argument put for the "expat" Interveners at the hearing was that by the 1972 Act, Parliament had conferred a legislative competence on the EU institutions, and in that way had changed the constitutional settlement in the UK.

In support of the contention that when passing the 2015 Act Parliament well knew of the Article 50 procedure for leaving the European Union if that was voted for in the referendum, he said that Parliament had previously dealt with it when the Lisbon Treaty was included in domestic law by the 2008 Act, and he took the court through the legislation dealing with the European Union and its predecessor, namely: In further submissions for the government, the lead claimant's primary argument was said by Treasury Counsel (James Eadie) to be that it is not open to the executive to use the prerogative power in such a way as to affect or change current economic law, principally statute law;[31] but the government contended that the leading case Attorney General v De Keyser's Royal Hotel meant that the question about the use of the royal prerogative depended on Parliament's legislative intention.

The decision was against the government's contention that the Crown's prerogative allowed giving Article 50 notice, and the court would later decide on the form of declaration it would make.

[41] The court held that the Government had no power to trigger notification under article 50 of the Treaty on European Union (TEU), because it would remove a series of rights created by Acts of Parliament.

The Daily Telegraph commented that the High Court ruling increased the prospect of an early general election,[50] while the Financial Times and The Guardian reported the case as a "blow" or a "setback" to the British government plans.

[54] Shadow Justice Secretary Richard Burgon condemned personal attacks from newspapers on the judges, describing them as "hysterical", and called on Lord Chancellor Liz Truss to speak out and protect them.

[55] The oath of office (prescribed by the Constitutional Reform Act 2005) obliges a Lord Chancellor to respect the rule of law and defend the independence of the judiciary.

[57] The oath of office for judges obliges them to "well and truly serve" the Queen and "do right to all manner of people after the laws and usages" of the realm "without fear or favour, affection or ill will".

[67] Intervening for the Scottish government, the Lord Advocate stated as background that the UK "acceded to the constitutional order of the Communities" when joining on 1 January 1973[68] and argued that "[t]he purported giving of notification under Article 50 TEU by unilateral act of [the British government] would be unlawful" because it would (inter alia) Before the hearing, the Supreme Court invited the public to view video footage of the entire proceedings, and provided on its website a page headed "Article 50 'Brexit' Appeal" with multiple links, giving a brief explanation of the issues to be considered and other information, and stating that in addition to live video feeds and 'on demand' catch-up video of each court session, transcripts would be available at the website on a half-daily basis (morning session by 4 pm, afternoon session around 7 pm).

[70][71][72] The government's written case, prepared in advance of the hearing of the appeal, and subscribed by the Attorney General for England and Wales and the Advocate General for Scotland,[73] included footnotes referring to legal comment, critical of the High Court's judgment, on pages of UK Constitutional Law Association and two other websites: The Daily Telegraph commented that ministers had accused the judges of relegating the referendum vote to a footnote, and backing the claim that a vote from the House of Commons and House of Lords was now needed before UK and EU talks began.

Others listed as participating in the hearing were: The Court published a table setting out the time allotted for the hearing of the oral arguments of the parties' advocates in the four days, Monday 5 to Thursday 8 December:[71] Before calling on the Attorney General to open the case for the government as Appellant, the Supreme Court President stated the justices were aware of the strong feelings associated with the many wider political questions surrounding the United Kingdom's departure from the European Union, but the appeal was concerned with the legal issues, and their duty was to consider those issues impartially and decide according to the law.

[78] The Appellant's submissions, apart from devolution issues to be addressed later by the Advocate General for Scotland,[79] were summed up on the morning of the second day in a series of points: Following on, the Advocate General for Scotland ended his oral submissions for the Appellant by saying that if an exercise of the royal prerogative to take the UK out of the EU were seen as an abuse of power after the 1972 Act, there could be no such abuse after the Referendum Act 2015 and the result of the referendum was known: "It is simply a question of whether it would be proper and appropriate for the executive to exercise the prerogative in particular circumstances, and the circumstances that we have to address are those which exist today in light of the 2015 Act, which is of considerable constitutional importance and the decision made in the referendum, knowing that if Parliament wanted to intervene and limit the exercise of that prerogative right, it is free to do so and has chosen to remain silent.

[84] In response to submissions of parties opposing the appeal and questions put by the Justices, it was said for the government that the question before the court was about "the present state of the division of responsibility between our pillars of state, legislative, executive, and indeed judicial, and that demands a current answer and not a historic one"; and that parliament's legislation was to implement British treaty obligations, not to control the government's exercise of the royal prerogative on the international plane.

[85] Closing the hearing, the Court President said that the appeal raised important constitutional issues, and the Justices would take time to give full consideration to the many arguments presented to them, orally and in writing, and they would do their best to resolve the case as quickly as possible.

Had the Bill which became the 1972 Act spelled out that ministers would be free to withdraw the United Kingdom from the EU Treaties, the implications of what Parliament was being asked to endorse would have been clear, and the courts would have so decided.

For these reasons, we disagree with Lloyd LJ’s conclusion in Rees-Mogg in so far as he held that ministers could exercise prerogative powers to withdraw from the EU Treaties.... 90.

The act's long title is To Confer power on the Prime Minister to notify, under Article 50(2) of the Treaty on European Union, the United Kingdom's intention to withdraw from the EU.

The act's two sections are to confer on the Prime Minister the power of giving the notice that the Treaty requires to be given when a member state decides to withdraw.