The Supreme Court has today ruled, by an eight to three majority, that an Act of the UK Parliament is required before the UK Government can give the notice under Article 50 of the Treaty on European Union that begins the process of the UK leaving the EU. This was a defeat for the UK Government, which had maintained that it could rely on Royal Prerogative powers to give that notice without requiring legislation.

Much will be written by others on the Supreme Court's decision on that main point, though it does not add a great deal to the earlier decision of the High Court of England & Wales on which we wrote previously. One new point is that the Supreme Court has confirmed that the UK Parliament's approval must be given by way of legislation. While that was the logic of the High Court's decision, it had not previously been confirmed expressly. However, paragraph 122 of the Supreme Court's decision noted that it is for the UK Parliament to decide what form that legislation should take, noting the submissions that had been made to the effect that a very brief statute would suffice. The UK Government has confirmed that a "straightforward" Bill to authorise the Article 50 notice will be introduced within days.

The other significant constitutional issue that was before the Supreme Court, and with which the High Court had not had to deal, was raised in (among other places) the intervention by the Lord Advocate on behalf of the Scottish Government. That intervention asked the Court to declare that the Scottish Parliament's consent to the Article 50 legislation should be sought, in line with the constitutional convention that the UK Parliament will not normally legislate in relation to devolved matters without that consent (generally known as the 'Sewel convention'). The Lord Advocate argued that the effect of Brexit on devolved areas of law, and on the Scottish Parliament and Scottish Government's powers in devolved areas, meant that the convention would be triggered. See here, here and here for more on the Lord Advocate's argument.

The Supreme Court unanimously declined to agree with the Lord Advocate's argument, but reports so far have not necessarily been quite right in identifying the basis for their decision, which does need a little unpacking.

One important point to note is that, while the Court did state that the Scottish Parliament does not have "a legal veto" on the UK's withdrawal from the EU (at paragraph 150), it was doing no more than acknowledging a point the Lord Advocate had already conceded. Reports and statements implying that the Court rejected an attempt by the Lord Advocate to establish a veto for the Scottish Parliament over any Article 50 legislation are therefore wide of the mark. Rather, the Lord Advocate's argument encouraged the Court to declare that Article 50 legislation would engage the Sewel convention. If the Court had been willing to do that, it would then be politically difficult for the UK Parliament to proceed without asking for consent, or notwithstanding a refusal of consent.

While the Court declined to go along with the Lord Advocate on that, it also did not say expressly that the Scottish Parliament's legislative consent would definitely not be required. Instead, it avoided that substantive question by saying that it is not for the courts to rule on the application or even the scope of conventions such as the Sewel convention. The judgment explained that conventions and their enforcement belong in the political arena, and if a convention is departed from it is not for the courts to enforce compliance with it. In other words, conventions are not justiciable.

The addition of section 28(8) to the Scotland Act 1998, which "recognised " that the UK Parliament "will not normally" legislate "with regard to devolved matters" without consent did not affect that general proposition. The Court, agreeing with the position stated by the UK Government at the time that sub-section was amended into the Scotland Act, said that its existence was a signifier of the political significance of the convention but did not make it justiciable.

Accordingly, at its most basic level the Supreme Court's judgment put the question of the Scottish Parliament's consent back into the political arena, Because it did not make any definitive statement on whether the Sewel convention would be engaged, it leaves some scope for continued argument on the point (if only because the judgment ensures no court can ever issue a final, authoritative decision on the question). However, certain aspects of the Court's decision suggest the judges in fact took the view that legislative consent would not be required under the convention.

In particular, paragraph 130 stated that "the devolved legislatures do not have a parallel legislative competence in relation to withdrawal" from the EU. Lord Neuberger, delivering a summary of the judgment from the bench, added that "relations with the EU are a matter for the UK Government". These statements would seem to favour the UK Government's position that legislating to trigger Article 50 would not be legislating "with regard to devolved matters", notwithstanding that it may have (potentially significant) consequential effects in devolved areas.

Paragraph 130 also noted that "the removal of the EU constraints on withdrawal from the EU Treaties will alter the competence of the devolved institutions unless new legislative consents are introduced. In the absence of such new restraints, withdrawal from the EU will enhance the devolved competence". This statement could appear to support an argument for seeking legislative consent, on the basis that the UK Parliament has not normally legislated to change the competence of the devolved institutions without consent. However, it is notable that the Supreme Court limited its definition of the Sewel convention only to legislating in devolved areas, and declined to extend it to include altering the limits of devolved competence notwithstanding the Lord Advocate's attempts to argue for a wider 'legislative consent convention' that included both matters. This suggests that the Court may only view the practice of seeking consent before amending devolved competence as just that - a practice, which does not rise to the level of a convention. That could again reinforce UK Government arguments that there is no constitutional convention requiring it to seek the consent of the Scottish Government before legislating to invoke Article 50.

However, the Supreme Court's decision is unlikely to result in the issue of legislative consent disappearing. It leaves enough scope to allow continued arguments at the political level, and the First Minister had already indicated before today's decision that she intended to hold a vote in the Scottish Parliament on triggering Article 50 whatever the outcome of the case. The Scottish Parliament will therefore be expressing its view in any event. Today's ruling does make a difference in that the UK Government will feel less political pressure from such a vote than they would if the Supreme Court had said Scottish Parliament approval was required under the constitution. However, there has been no shortage of political argument between the Scottish and UK Governments prior to today's decision, and that is highly unlikely to change.