HOLYROOD’s Brexit Bill is “perfectly practical,” the Lord Advocate told the UK Supreme Court and was backed up in his arguments by legal officers from Wales and Northern Ireland as the Scottish Government continued its unprecedented constitutional battle with Whitehall.

In the second and final day of the court hearing in London, James Wolffe rejected the UK Government’s main contention that by passing Edinburgh’s Continuity Bill the Scottish Parliament was in some way trying to cut across Westminster’s authority in international relations.

He argued the Scottish Bill was not incompatible with EU law, which was based on the principles of “legal certainty and continuity”.

The Lord Advocate said the Holyrood legislation only impacted on the “domestic legal order” and so could not affect the UK's negotiations with Brussels.

Crucially, Mr Wolffe told the court that the Scottish legislation had been passed by MSPs several months before the UK Government’s flagship EU Withdrawal Bill became law, noting: “It was not intended to modify the UK Bill and could not do so.”

On the first day of the hearing, Lord Keen of Elie, the Advocate General for Scotland, leading for the UK Government, argued 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.

But the Lord Advocate’s points were supported by Michael Fordham for the Welsh Government, who claimed the UK Government’s contention that the Scottish Bill would cut across Whitehall’s reserved powers on international relations was “wrong and incoherent”.

He said the UK’s argument that devolved administrations could not legislate in devolved areas currently managed by the EU because they touched on international treaty negotiations was an “extravagant claim which has very alarming logical implications".

He explained that this would have the effect of restricting the powers of devolved parliaments even after Brexit and EU law was transferred onto the UK Statute Book.

John Larkin, the Northern Irish Attorney General, also argued that the Scottish Bill was “within the legislative competence of the Scottish Parliament".

But in response Lord Keen suggested it was “unjustified” for devolved governments to expect that all devolved powers should be handed over after Britain left the EU, because at the time the devolution settlement was drawn up 20 years ago, no one could have predicted Brexit would have happened.

He said it was “not open to the Scottish Government and the Scottish Parliament to assume that no new legislative constraints” would be introduced because of Britain leaving the EU.

He claimed the Continuity Bill was “fundamentally inconsistent” with the purpose of the UK Act and that the confusion over which Parliament had jurisdiction over which European regulations would result in the undermining of the UK Government’s Brexit strategy.

Seven judges of the Supreme Court, including two Scottish justices, will now spend the summer considering their ruling; expected at some time before Christmas.

If they were to find for the Scottish Government, then this would mean the Continuity Bill would receive Royal Assent and become law.

However, this would mean that there would be two competing pieces of legislation on the Statute Book, which would likely result in the matter returning to the Supreme Court to determine which Act took precedence.

Alternatively, it could transpire that the judges determine some parts of the Continuity Bill are within Holyrood’s competence and some are not, which would mean MSPs having to amend the legislation appropriately.