This is the question that has been central to debates in the House of Commons over the past week and to the legal challenge currently before the High Court.
The legal challenge being brought by Gina Miller and Dier Tozetti dos Santos as principal claimants against the Secretary of State for Exiting the European Union, focuses on the specific issue of whether Parliamentary authority is required before Article 50 of the Treaty on European Union (the formal exit mechanism for a withdrawal by a Member State from the European Union) can be triggered. Since the June referendum, there has been a continuing debate as to what the UK needs to do for the purpose of “its own constitutional requirements” in order to serve notice under Article 50.
In what is one of the most important constitutional cases to come before the Courts, the Government is arguing that it has a unilateral right to trigger Article 50 using residual Crown prerogative powers in relation to the making or breaking of international treaties. The Claimants argue that prerogative powers cannot be used by the Government to override existing Acts of Parliament (e.g. the European Communities Act 1972) unilaterally or to remove fundamental rights of UK citizens currently existing under EU law. In September, the House of Lords Constitution report published in mid-September (see September blog post) came down on the side of the Claimants.
The Government is also arguing that the legal challenge should be dismissed on the basis that the relief sought by the Claimants is constitutionally impermissible as the Court would be “trespassing on proceedings in Parliament” by requiring the Secretary of State to introduce legislation into Parliament to trigger Article 50 in order to give effect to the referendum result. The Claimants counter that the Court does have authority to make the order sought on the basis that the prerogative forms part of the common law and so only the Court can determine whether or not a particular prerogative exists, the extent of such power and how such power must be exercised.
Judgment is expected to be given within a short timeframe. From a practical perspective, the outcome of the case may impact on the stipulated deadline set by the Government for the triggering of Article 50 of the end of March 2017. Given the importance of the decision, it has also already been indicated at an earlier hearing in July that, in the case of an appeal, it will be appropriate to consider what is called a “leapfrog” appeal. This means any appeal would bypass the Court of Appeal and jump straight to the Supreme Court for a final decision, likely to be given by the end of this year.
The cross-party debates in the House of Commons last week looked at the wider question of Parliament’s role in the overall Brexit timetable and in particular the proper role of Parliament as the country’s elected representatives in scrutinizing and holding the Government to account on the terms of the UK’s withdrawal from the EU. The first debate on Monday October 10, followed a statement given to the House of Commons by the Secretary of State for Exiting the European Union, David Davis, on “next steps in leaving the European Union”. The second debate on Wednesday October 12 followed the tabling of a motion by the Labour opposition party for a full Parliamentary debate on the proposed terms for Brexit negotiations and a letter sent by Shadow Secretary of State, Keir Starmer, and Shadow Foreign Secretary, Emily Thornberry, to David Davis, attaching 170 questions for the Government to answer in the 170 days leading up to the stated deadline set by the Government for Article 50 to be triggered, to emphasize the complexity of the withdrawal process.
By the end of last week, the Government had confirmed that Parliament would be given the opportunity for a further debate before the triggering of Article 50, but the timing of such debate remains to be clarified and the Government appears for the time being to have resisted calls for any vote by Parliament which might in any way fetter the Government’s proposed opening terms for Brexit. This outcome of the current legal proceedings could alter that and last week was perhaps only the opening round in the debate between Government and Parliament as to Parliament’s proper constitutional role in the overall Brexit process.
 Together with other interested parties including the People’s Challenge group, a group backed by a crowd-funding campaign.
 Article 50(1), Treaty on European Union: “Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.”