Over the course of six days in this past two weeks, the Divisional Court of England and Wales and the Inner House of the Court of Session of Scotland considered two very similar cases.

The cases concerned the same set of facts, in relation to the same exercise of prerogative power, with similar arguments from both sides and the same authorities cited.

Ultimately, however, the decisions differed over the simple but crucial question of whether the decision to advise Her Majesty the Queen to prorogue Parliament was one which the courts could review or not.

Indeed this issue as to the 'justiciability' of the claim was as far as the Divisional Court went, and it did not provide an opinion on whether the decision itself was lawful - contrary to the headlines and press coverage that subsequently reported it.

The Scottish court, however, did offer a decision on the lawfulness of the decision, and in doing so came to a different conclusion to the English court - in addition, of course, to those of the Outer House of the Court of Session - on the question of whether it could make such a determination in the first place.

It is in respect of this question of justiciability that the real difference between the decisions lies. A difference that the Supreme Court will now be required to settle.

Prerogative Powers and Justiciability

On the question of prerogative powers and when they can be subject to judicial review, both courts began with agreement on the underlying legal principles. Both recognised the generally accepted position that prerogative powers by their nature are not immune from judicial review, and further that the question of whether such decisions are reviewable will depend on the subject matter of the decision rather than the source of the power which underpins it.

It is at this point that a subtle divergence emerges. The Divisional Court noted the consistent refusal of courts to intervene in respect of subject matter which involves 'high policy' or is 'political', citing the inability of courts to resolve such questions and the lack of legal criteria against which to judge them.

The Court of Session pointed to the same generally accepted position, citing many of the same authorities, but crucially Lord Carloway specifically tied this accepted position to decisions based on 'legitimate' political considerations. Lord Carloway continued, noting that had the reasons for the Prime Minister's decision to advise the Queen to prorogue Parliament been based upon 'legitimate' political considerations, including a desire to see Brexit delivered, then the decision would not be justiciable.

Lord Carloway went on to conclude that this was not the case in the present circumstances, and that the true reasons for the decision were to reduce the time available to Parliament to scrutinise the Government's approach to Brexit.

Lord Brodie concurred, noting that a Government that resorts to procedural manoeuvrings in order to achieve its objective does not automatically become liable to judicial review. However, where the political manoeuvres are 'so blatantly designed "to frustrate Parliament" at such a critical juncture', then a court may legitimately intervene.

In short, Lord Carloway and Lord Brodie's decisions on justiciability seem to be predicated on a prior decision as to the lawfulness of the decision.

For his part, Lord Drummond Young went further, stating that the rule of law requires that any act of any public institution, including the Executive, must be liable to judicial review to ensure that it is within the scope of the legal power exercised.

The approach taken by the Scottish court is a notable departure from that taken by the Divisional Court, where Lord Chief Justice Burnett concluded - based on the same authorities - that a decision on unlawfulness could not be reached, because the decision itself was not justiciable. Indeed, the Divisional Court explicitly rejected the claimant's suggestion that it should look first at whether there had been a public law error before considering the question of justiciability.

Lord Burnett went even so far as to suggest that the true reasoning for the prorogation may well have been, in the words of Lord Carloway, to 'stymie' Parliament, but that this was an inherently political decision in any event and the courts did not have the means to review it.

The essential problem was said to be the absence of any judicial or legal standards by which to assess the legality of otherwise of an essentially political action on the part of the Government. In particular, in this case, it would be impossible for the court to assess whether the duration of the prorogation was excessive by reference to any measure.

It is interesting to note that on this issue the Government conceded before the Scottish court that in certain circumstances prorogation would be justiciable - the examples given being if Parliament was prorogued for two years or if prorogued by a Government that had just lost a general election. In effect too long a prorogation could give the court the power to intervene. That concession immediately moves the argument on to different territory and was recognised as important in the reasoning of Lords Brodie and Drummond Young.

Although it might have been hard to resist making when pressed by the bench during oral submissions, the concession does detract from the purity of the Government's argument. We shall see how it might be finessed in argument before the Supreme Court.

Parliamentary Sovereignty

Another interesting difference between the decisions of both courts is in respect of the concept of Parliamentary Sovereignty. The challengers on both sides of the border wasted few superlatives when it came to explaining the role of this key pillar of the UK constitution. But whereas the concepts by the petitioners were largely taken at face value in the Scottish court, the English court took exception to the claimant's construction.

In particular, Lord Burnett considered that the claimant not only formulated this concept in a manner which went beyond what is generally accepted, but did so in order 'to invite the judicial arm of the state to exercise hitherto unidentified power over the Executive branch of the state in its dealings with Parliament'.

Both cases will now be heard by the Supreme Court. The resulting judgment will be an important authority on the boundaries between the three branches of power in the UK's constitutional arrangements.