THE UK and Scottish Governments will today lock horns in an unprecedented constitutional battle at the UK Supreme Court in London over whether or not the SNP administration’s own Brexit Bill is legally competent.

Led by Baroness Hale, the Court’s President, the two-day case will be heard by seven judges, including two Scots, Lord Reed and Lord Hodge. A judgement is expected by the end of the year.

Were the Supreme Court to rule in the Scottish Government’s favour, then its Continuity Bill would receive Royal Assent and become law.

However, this would mean that there would be two competing laws – the Continuity Act and the EU Withdrawal Act - on the Statute Book, which is likely to mean that the matter would return to the Supreme Court to determine which law took precedence.

Given the UK is due to leave the EU on March 29 2019, then any constitutional wrangling before the judges would have to occur quickly in the New Year.

In March, the Scottish Government proposed and won Holyrood’s backing for its own Brexit bill, because First Minister Nicola Sturgeon and her colleagues opposed MSPs consenting to the UK Government’s Withdrawal Bill, believing it to be a “power-grab” on devolved powers; a claim forcefully denied by Whitehall.

Scotland’s chief legal officer, the Lord Advocate, James Woolfe, has set out the SNP administration’s case in a written submission to the court, explaining that it believes Scotland‘s “system of laws should continue to operate on the day after the UK leaves the EU as it did on the day before” to ensure consistency and predictability for Scottish citizens and businesses.

“The Scottish Ministers also consider that it should remain possible for devolved legislation to continue to be updated or aligned with new EU law where that is appropriate. The purpose of the Bill is therefore to ensure the greatest possible continuity of law on and after the day that the UK leaves the EU.”

Leading for the UK Government will be Lord Reed of Elie, the Advocate General for Scotland, who will argue that, despite Holyrood’s endorsement, the Continuity Bill is not within the Scottish Parliament’s competence; a view shared by Ken Macintosh, Holyrood’s Presiding Officer.

Ahead of the court case, David Mundell, the Scottish Secretary, who previously described the legal clash as “not a big deal,” pointed to Mr Macintosh’s position, saying: "Given the view of the Scottish Parliament’s Presiding Officer that the Continuity Bill was not within the legal scope of the Parliament, we believe it is important to ask the Supreme Court to provide absolute clarity.

“The legislation which set up the Scottish Parliament anticipated such a situation and we are simply following the legal process set out in the 1998 Scotland Act.”

A Scottish Government spokeswoman said: “The Lord Advocate has set out in his written case that it is within the powers of the Scottish Parliament to prepare for the consequences for devolved matters of UK withdrawal from the European Union.

“As this is a live matter before the Court it would be inappropriate to comment further at this time.”