Professor Richard Ekins is Head of the Policy Exchange’s Judicial Power Project and Professor of Law at the University of Oxford.

Tomorrow, the Supreme Court will hear argument about whether it should declare unlawful the Government’s advice to Her Majesty to prorogue Parliament, and whether, in consequence, to declare to be null and of no effect the prorogation that followed on 10th September.

This is an important question, the answer to which turns on the nature of our constitution. I examine it in a Policy Exchange paper published today, Parliamentary Sovereignty and the Politics of Prorogation, and conclude that the Supreme Court should uphold the judgment of the Divisional Court, which rightly recognised that the prerogative power to prorogue is not subject to judicial control.

It should reverse the judgment of the Inner House of the Court of Session, which wrongly asserted a jurisdiction to control the exercise of the power on the grounds that the Government acted for an improper purpose. If the Supreme Court was to hold that the advice to Her Majesty was unlawful, it should nonetheless recognise that it has no authority to quash the prorogation of Parliament that has already taken place. This was a proceeding in Parliament and of Parliament which courts cannot lawfully question.

The prerogative power to prorogue Parliament is exercised by Her Majesty on the advice of the Government, which is accountable to the House of Commons. The power to prorogue is an important feature of the Westminster constitution, in the UK and in other related systems, and enables the Government to control the timing and length of parliamentary sessions, a power which it is free to use to manage parliamentary business.

The Government is responsible to the Commons, and eventually to the electorate, for its use of this power. It is open to political criticism if it is seen to misuse the power, but unless and until confidence is withdrawn it is entitled to use it. The only circumstances in which it might be open to Her Majesty to refuse a prorogation would be where prorogation was sought to remain in office after a vote of no confidence had been lost.

The Westminster constitution is framed by constitutional convention and practice. The law recognises a sharp distinction between convention and law, and forbids courts from adjudicating disputes about the former, as the Supreme Court itself recognised clearly in Miller (No 1) (the legal challenge brought by Gina Miller with the support of Sir John Major).

Judicial control of the prerogative to prorogue is not justified or required by the fundamental rule (or principle) of parliamentary sovereignty. Proroguing Parliament in no way flouts parliamentary sovereignty. Parliamentary sovereignty is not set aside during a prorogation any more than it is after a dissolution. It is wrong to think that this prorogation bypasses Parliament or turns the constitution on its head.

The House of Commons had an opportunity to withdraw confidence before prorogation and did not act. If the constitution has been turned on its head in recent weeks, it is not by virtue of this prorogation but by the procuring of legislation to force the Government to depart from its central policy and to apply for an Article 50 extension, and by the refusal of the House of Commons to withdraw confidence in the Government or to permit an early election to be held.

The Court of Session reasoned that the Government acted for an ‘improper purpose’, seeking to “stymie” Parliament. However, the courts are not free, as a matter of our law, to apply the ordinary grounds of judicial review, including the rule that one cannot act for an ‘improper purpose’, to the prerogative power to prorogue or to advise about how the prerogative should be exercised.

The courts are not well-placed to decide what is or is not a proper purpose for prorogation. The Court of Session wrongly took upon itself to decide that the Government illegitimately sought to avoid scrutiny rather than legitimately sought to manage parliamentary business and to improve the UK’s negotiating position in relation to the EU. The question of how the power to prorogue Parliament should or should not be used is a political question over which the courts have no jurisdiction.

The Court of Session in effect wrongly departed from the legal rule that courts should not enforce, or invite argument about, constitutional practice and convention. The Government was free to decide to prorogue Parliament, and it is rightly answerable to the House of Commons and to the electorate for this decision. It should not be answerable to the courts for this action.

The Court of Session’s judgment wrongly interferes in a proceeding of Parliament, first by declaring unlawful advice about how the prerogative was to be exercised and second by declaring that the prorogation that followed was null and of no effect.

The Bill of Rights 1689 forbids judicial interference in parliamentary proceedings – and prorogation is a proceeding of Parliament, which brings to an end one session of Parliament and makes provision for the next to begin. The Supreme Court should reverse the Court of Session and uphold the Divisional Court, thereby helping to arrest a worrying trend of judicialising political questions and parliamentary processes.