THE crucial constitutional case in which the Westminster Government is challenging the Scottish Parliament’s Continuity Bill, in an attempt to grab back powers that are returning from the European Union after Brexit, got under way this morning at the UK Supreme Court (UKSC) in London.

The Advocate General for Scotland, Lord Keen of Elie QC, the former Scottish Conservative chairman, made the initial submission on behalf of himself and the Attorney General Geoffrey Cox.

The Lord Advocate of Scotland, James Wolffe QC will later present the submissions on behalf of the Scottish Parliament. (For sake of consistency in our own text we will refer to the UK law officers and the UK Government as Westminster, the Scottish Parliament as Holyrood and the Continuity Bill as the Scottish Bill – as it is called by Lord Keen – and the EU Withdrawal Act as the UK Act, again as referred to by the Advocate General. All other descriptions are verbatim from the court.)

Seven Supreme Court justices are hearing the case brought by Westminster against Holyrood. They are led by Lady Hale, the UKSC President, and include two Scottish judges, Lord Reed and Lord Hodge.

In brief, they are being asked to decide whether the Scottish Bill is within the competence of the Scottish Parliament. There are, however, several complex issues of law at stake in this, the first case of its kind involving Westminster against Holyrood. Earlier this morning the UK law officers published their written submission to the UKSC, a week after Scotland’s Lord Advocate did so on behalf of Holyrood. The Welsh and Northern Irish law officers also published their submission a week ago, backing the Scottish case.

A key element of the latest written submission stated: “The Scottish Bill establishes a new and far-reaching legal framework in Scotland derived from, and relating to, the EU and EU law. It legislates as if the European Communities Act no longer applied, for a context in which there is a new relationship between the UK and the EU, but without any understanding of what the nature of that new relationship is to be or how it is to be given effect in domestic law.

“The new architecture the Scottish Bill purports to create in particular is that of a new and substantial body of Scots law (as opposed to EU law) and power to fix and modify that body of law.

“The Scottish Bill purports to adopt powers to continue to give effect to EU law, requires the Scottish Ministers to have regard to EU law in certain areas after withdrawal including subsequent changes in that law, and to restrict the ability of UK Ministers to legislate (s.17).

“It is an Act of the Scottish Parliament of unparalleled scope and seeks to create a broad framework for current and future law derived from the EU, at a time when, and in a context where, the future relationship of the UK and the EU remains under negotiation and in transition.”

Later in the written statement is the nub of Westminster’s case: “The importance of adopting a consistent approach to the effect of withdrawal across the UK as a whole is underlined both by the existence and detail of the UK Bill, and the close but not exact parallels adopted in the Scottish Bill.

“Whatever the final terms of the UK Bill, it is unquestionable that [the UK] Parliament will legislate for the effects of withdrawal precisely because it is a matter of major constitutional importance in which the UK as a whole has an interest.

"In simple terms: legislation addressing the effect of withdrawal from the EU, in particular making provision for the continued application of established law in areas currently within the competence of the EU, is a matter for Parliament and not the devolved legislatures.”

In his opening remarks, Lord Keen said the Scottish Bill “creates a separate and novel body of law” and “fundamentally undermines” the core purpose of the UK Act.

He also said that the Scottish Bill is "fundamentally inconsistent" with the UK Act passed by MPs.

The Holyrood bill includes several provisions not mirrored in the Westminster one, such as retaining the EU's Charter of Fundamental Rights and giving ministers the power to suggest EU laws to "keep pace" with even after Brexit.

Lord Keen said it was "perfectly clear" that the Scottish bill was "directly inconsistent with the UK Act at the most basic of levels", saying "the two simply cannot stand together".

He told the court that this would create "dual and inconsistent regimes" within the UK, which would "directly frustrate the purpose" of the Withdrawal Bill, which was to create a "single cohesive body" of EU laws retained after Brexit.

He also argued that the Continuity Bill could have a bearing on international relations, a field reserved to the UK parliament. He said that "withdrawal from the EU is a matter for the UK parliament, and the devolved administrations do not have a parallel legislative competence" in this area.

Referring to the case taken by Gina Miller against the UK Government over Parliamentary approval for Article 50, Lord Keen quoted the Supreme Court’s own judgement that the withdrawal from the EU is a matter for the UK Parliament and the devolved legislatures in the UK do not have a parallel legislative competence in this matter whereas the Sovereign Parliament is "untrammelled" in what it can do.

He said: "The UK Parliament is sovereign, the Scottish Parliament is not."

The Lord Advocate is expected to argue either later today or more likely tomorrow that Westminster is "wrong" in its submissions.

The case continues and two days have been set aside for the UK Supreme Court to hear evidence.