THE Scottish Government’s Brexit Bill could “undermine” the UK Government’s negotiation strategy with the EU, the UK Supreme Court has been told.

As the unprecedented constitutional battle between London and Edinburgh got underway before seven judges over whether the Scottish Continuity Bill is within the legal competence of Holyrood, Lord Keen of Elie, the Advocate General, leading for the UK Government, also argued that because Theresa May’s flagship EU Withdrawal Act was a “protected enactment,” it could not be modified but that, he stressed, was what Nicola Sturgeon’s legislation sought to do.

“The entirety of the Scottish Bill impermissibly modifies the UK Act and is accordingly outside competence,” he declared, explaining that as far as the UK Brexit Act was concerned no Holyrood legislation could “modify” it.

MSPs passed the Continuity Act because they could not agree to support the UK Government’s Brexit legislation, claiming it was a “power-grab” on devolved powers by Whitehall. The aim of the Scottish Bill, which has yet to get Royal Assent and become law in its own right, is to plug any gaps in Scottish laws and protect devolved powers post Brexit.

The seven judges of the Supreme Court, who include two Scottish justices, are being asked to rule on whether the Scottish bill is constitutional and "properly within devolved legislative powers".

As intricate legal arguments began in the two-day hearing, a written submission by Lord Keen pointed out how the Scottish Bill was passed "without knowledge" of the outcome of the negotiations between the UK and Brussels and so "pre-empts them".

It went on: "The effect of what the Scottish Bill does is to make provision for the future relationship with the EU and EU law when that relationship is under negotiation," consequently it says this "could serve to undermine the credibility of the UK's negotiation and implementation strategy in the eyes of the EU".

Lord Keen told the judges that the Holyrood legislation was “plainly and directly inconsistent with the UK Act at the most basic of levels” and that its “mechanism directly frustrates the purpose of the UK Act”.

He explained that “any reasonable consideration” of the Scottish Bill and the UK Act showed the former sought to modify, ie undermine, the latter and so the “two simply cannot stand together” as they produced two “dual but inconsistent regimes” in respect of retained EU law.

The UK Government law officer gave the Human Rights Act as another example of a protected enactment.

“You might not seek to amend or repeal any term of the Human Rights Act but you might by way of the Scotland Bill seek to introduce some parallel human rights provisions in Scotland which impacted upon the UK-wide provision for human rights and thereby modified the protected enactment and undermined the very purpose of it being a protected enactment.”

Lord Keen added: “We consider that given the UK Act is a protected enactment…it is quite clear the Scotland Bill will operate in its terms to modify the UK Act as it applies in Scotland by introducing this dual but inconsistent regime and, consequently, it is beyond the competence of the Scottish Parliament.”

James Woolfe, who is due to begin the case for the Scottish Government’s Bill later this afternoon, concluding tomorrow, has said in his written submission: “The purpose and effect of the Scottish Bill is to promote legal certainty by making provision for the continuity within the domestic legal system of existing EU-derived law upon and following withdrawal.

"Regardless of any treaty on the future relationship which may be entered into between the UK and the EU, there is a need to provide for legal certainty and continuity when the UK leaves the EU in March 2019, and that is the purpose and effect of the Scottish Bill."

Submissions will also be made by the Attorney General for Northern Ireland and the Counsel General for Wales as "interested parties".

The ruling of the court is expected before the end of the year.

If the judges were to find in favour of the Scottish Government’s bill, then it would pass onto the Statute Book. However, this would create a constitutional dilemma as there would be two conflicting pieces of legislation, which would give authority to two governments to exercise powers post Brexit.

In such a scenario, the matter is likely to go back to the Supreme Court for it to judge which piece of legislation took precedence.