Holyrood's approval would be required if Westminster attempted to change the parliament's powers the way Brexit will, the Scottish Government's most senior lawyer has told the Supreme Court.

James Wolffe QC, the Lord Advocate, also said he was not arguing that the Scottish Parliament had a veto over leaving the European Union.

But Holyrood's consent was needed before exit talks could begin because Brexit would bring "significant changes" to the areas under its control, he said.

READ MORE: SNP Transport Minister Humza Yousaf blames marriage split for driving without insurance

MSPs are likely to vote against triggering the official process to leave the EU, potentially creating a constitutional crisis, if they get a chance.

The UK Government is appealing a ruling by the High Court that MPs must have a vote on Article 50 of the Lisbon treaty.

This kicks off an official two-year period of 'divorce' negotiations between the UK and the other 27 EU members states.

On day three of the landmark court case, Scotland’s most senior law officer argued that a convention, known as the Sewel convention, requires that the Scottish Parliament consents before Westminster can legislate on a a devolved issue.

He said: "Can I make clear that I do not assert that the Scottish Parliament has a veto on the decision to withdraw the United Kingdom from the European Union."

"What I do say is that the question of whether the Scottish Parliament consents or does not consent to the effects of withdrawal with regard to devolved matters is, by virtue to the legislative consent convention, a matter of constitutional significance."

He said that Brexit would mean some legislation passed by Holyrood would become "redundant".

He added: "If a bill were to come before the United Kingdom Parliament which changed the competences of the Scottish Parliament or the Scottish Government in these ways, let alone the other effects with regard to devolved competence, then such a bill would engage the legislative consent convention."

He also told the court argued that his case bolstered the argument for MPs to have a vote before the Prime Minister starts Brexit talks.

Only the Westminster Parliament, not the UK Government, can change Scotland's powers in the way being proposed, he argued.

That principle has been in place since 1707, he said, when the power to change Scotland’s laws was given to Westminster not to the Crown.

He added that it was a “fallacy” to claim that because an issue was reserved to Westminster then it could never require Holyrood's consent.

READ MORE: SNP Transport Minister Humza Yousaf blames marriage split for driving without insurance

The UK Government has argued that because Foreign Affairs is not a devolved area then MSPs should not get a vote on Article 50.

The UK’s four legislatures are "entitled to have a voice in the decision," Mr Wolffe told the 11 justices hearing the case.

But he faced tough questioning from a number, who questioned whether the consent of Holyrood was merely a convention or a legally binding requirement.

Earlier this week the Advocate General for Scotland Lord Keen argued that Westminster was sovereign and could not be prevented from legislating on any matter by Holyrood, including triggering Brexit.

The UK Government's top Scottish law officer said that MPs could "legislate at any time on any matter".

The convention that Westminster had to consult MSPs was just that, he said, a convention and not legally enforceable.

Lord Keen argued that the Scottish Government's case was "fatally undermined" because the area of foreign affairs is reserved to Westminster.

In the wake of the independence referendum, the Sewel Convention, named after former Scotland minister Lord Sewel, was included in the 2016 Scotland Act.

However, Lord Keen described it as merely a "political" device and a "self denying ordinance" for Westminster.

SNP MP Pete Wishart angrily described that argument as “know your place, Scotland."

Welsh and Northern Irish ministers are also intervening the case to argue that their devolved assemblies must also be consulted before Brexit talks can start.

Those who took the case originally argue that it was Westminster that created the rights UK citizens have as a result of EU membership and that only Westminster can take those away.

The Supreme Court will decide if Prime Minister Theresa May can use the so-called “royal prerogative” to trigger Article 50.

A ruling is not expected until January.

Last night the Scottish and UK Governments were at loggerheads over another issue after Scottish Secretary David Mundell claimed Scottish ministers had staged a climbdown on their call for full membership of the EU's Single Market.

Mr Mundell said that Nicola Sturgeon's administration now wanted full access instead, a charge emphatically denied by Scotland's Brexit minister Michael Russell.

READ MORE: SNP Transport Minister Humza Yousaf blames marriage split for driving without insurance

He said that the Scottish Government wanted the 'Norwegian option', which involves 'taking part' in the Single Market but without decision-making powers.

In a separate development a leading Brexiteer, Tory MP Bernard Jenkin said that Conservative ministers should offer Brussels a "zero/zero" deal to secure a quick Brexit.

Under his proposal other EU countries would get the chance to trade tariff-free with the UK in exchange for reciprocal arrangements with the continent.