"That this House approves Her Majesty's Government's decision of principle to join the European Communities on the basis of the arrangements which have been negotiated."
Prior to the referendum, the Prime Minister (David Cameron) reached an agreement with the EU which contained a number of important provisions but, in the event, the agreement figured only minimally in the referendum campaign and it lapsed when the referendum result was to leave.
2. A Member State which decides to withdraw shall notify the European Council of its intention......
The UK government appeared content to proceed on the basis that the referendum was the decision to withdraw. This “will of the people” argument has considerable political leverage even though the UK operates as “representative democracy”. Nevertheless, Parliament had not made the referendum result legally binding. In such circumstances, could the government have proceeded to implement the referendum result or was it necessary to seek Parliamentary authority?
In the events that happened, the constitutional processes for making a decision to withdraw from the EU came to play second fiddle to the question of notification.
Stage 2 - The second stage of Article 50 concerns notification of the decision to withdraw. A view was strongly held by the government and by many lawyers that prerogative powers relating to treaty making (and unmaking) could be used to give the notice to the EU without the need for specific parliamentary authority. This resulted in the legal challenge mounted by Gina Miller and Deir Dos Santos. Their argument was essentially that membership of the EU had given the citizen specific rights and those could only be removed by Parliament and not by the executive using prerogative powers. This was an argument founded on well-established law that the Royal Prerogative, whilst it embraces the making of treaties, does not extend to altering the law or conferring rights upon individuals or depriving individuals of rights which they enjoy in domestic law without the intervention of Parliament – see the speech of Lord Oliver in J.H. Rayner (Mincing Lane) Ltd v Department of Trade  2 AC 418 – (sometimes referred to as the Tin Council case).
The Secretary of State appealed to the Supreme Court of the UK which, exceptionally, sat with all 11 of its justices. The court dismissed the Secretary of State’s appeal by a majority of 8 to 3. The majority was Lord Neuberger, Lady Hale, Lord Mance, Lord Kerr, Lord Clarke, Lord Wilson, Lord Sumption and Lord Hodge. The minority was Lord Reed, Lord Carnwath and Lord Hughes. The judgments (majority and minority) are available via the Supreme Court website.
The Long Title of the Act is - "An Act 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." Section 1(1) of the Act states - "The Prime Minister may notify, under Article 50(2) of the Treaty on European Union, the United Kingdom’s intention to withdraw from the EU."
The Act did not impose a duty on the Prime Minister to notify the EU. That much is clear from the use of the word "may" rather than "shall" in section 1. The PM was given a "power" to notify.
Was there a decision?
Was there a decision to leave the EU made in accordance with UK constitutional requirements? This blog has long argued that it would have been legally preferable for Parliament - the UK's sovereign body - to have stated in legislation that a decision had been made - Please see posts of 27th June 2016 and 17th March 2017
The Notification Act did not explicitly state that Parliament had made a "decision" to take the UK out of the EU. The referendum was "advisory" and so cannot amount to a "decision" in law however much Ministers regard it as politically binding.
Nevertheless, it may be arguable that a decision in accordance with constitutional requirements was reached by a combination of the referendum result, subsequent debates in Parliament and the Notification of Withdrawal Act enacted as a result of the Miller and Dos Santos litigation,
For those who prefer clarity in such matters, all of this appears to be an unsatisfactory and untidy way of going about the making of vital choices for the nation's future.
The government handed withdrawal notice to the President of the EU Council on 29th March 2017. The government had “triggered” Article 50.
Question ii involved consideration of the so-called Sewel Convention - see Supreme Court's majority judgment at paras. 136-151. The court concluded - "..... we do not underestimate the importance of constitutional conventions, some of which play a fundamental role in the operation of our constitution. The Sewel Convention has an important role in facilitating harmonious relationships between the UK Parliament and the devolved legislatures. But the policing of its scope and the manner of its operation does not lie within the constitutional remit of the judiciary, which is to protect the rule of law."
The outcome was that, as a matter of law, the UK government did not need to seek the legislative consent of the Northern Ireland Assembly for any legislation relating to the giving of notice to the EU.
D] Professor Steve Peers (University of Essex), writing on EU Law Analysis (25th January 2017), commented that – “Despite many constitutional lawyers’ criticism of the technicalities of the judgment, in my view it at least fully expresses the traditional spirit of the UK constitution – and Parliament’s historic role in British political life.”
If Labour were to form the next government there would be a European Union Rights and Protection Bill in place of the Conservative Great Repeal Bill. This would ensure there is no detrimental change to workers’ rights, equality law, consumer rights or environmental protections as a result of Brexit.