The UK Government has been accused of trying to use the royal prerogative to "drive through" constitutional changes of a "seismic nature" following the Brexit vote.
Eleven Supreme Court justices heard the claim from a top lawyer representing the Welsh Government on the fourth and final day of a landmark case at the UK's highest court.
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Richard Gordon QC told the packed London court that what the Government was seeking to do was use the prerogative power "to drive through the most major constitutional change in our system at least since 1972".
That constitutional change was, he said, of a "seismic nature".
Attorney General Jeremy Wright QC. Photo: Yui Mok/PA Wire
Mr Gordon, presenting arguments on behalf of Mick Antoniw, the Counsel General for Wales, submitted that having a "treaty-making power" did not mean there was a power to "dispense with laws, or subvert statutory schemes or crucify human rights".
He told the justices: "We say use of the prerogative will dispense with law. If we are right about that, we win and the Government loses."
Mr Gordon, describing "what at heart these appeals are really about", told the court: "They are really about the proper distribution of power between Parliament and the executive in our society."
The Brexit vote had "split the UK into four parts", he said, describing it as almost "the most divisive political event that has happened over the last several decades".
In his opening remarks Mr Gordon told the justices: "On behalf of the Counsel General for Wales, who sits next to me, I want to make it clear that the position of the Welsh Government and the Counsel General is that the result of the referendum to leave the EU should be respected."
He added: "Wales is not here because it wants either to stop or to stall Brexit, or the implementation of Brexit. It is here precisely because the constitutional issues at stake go far beyond Brexit."
He argued that the case presented to the court by the UK Government was "flawed".
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Earlier, Scotland's most senior law officer, Lord Advocate James Wolffe QC, argued that an Act of Parliament was required to trigger Article 50 of the Lisbon Treaty to start the two-year process of withdrawing from the EU.
And he said the Scottish Parliament was also entitled to a voice on the issue, which involved the serious loss of EU rights for Scottish people.
The proceedings, which began on Monday amid a blaze of publicity around the world, will finish later with a ruling not being given until the new year.
The hearing at the UK's highest court follows a High Court ruling against the Government in November.
Three judges ruled that Theresa May lacked legal power to use the royal prerogative.
MPs have now voted in favour of the Government's timetable to trigger Article 50 by March 31 next year.
The case is due to end later today following final submissions from James Eadie QC for the Government.
Lord Pannick QC, the lawyer representing Gina Miller, arriving at The Supreme Court. Photo: Yui Mok/PA Wire
Manjit Gill QC, appearing for parents and children who are not UK citizens but European Economic Area (EEA) nationals, said it is important that Parliament, not Government ministers using the royal prerogative, should be involved in triggering Brexit.
Without safeguards, he said, EEA nationals could be told "to pack your bags and go" when the UK withdraws from Europe.
Read more: No Brexit deal possible without consent of Holyrood, says Lord Advocate James Wolffe
Mr Gill suggested nobody had thought at the time of drafting the EU Referendum Act 2015 which paved the way for the June referendum whether or not the prerogative would be appropriate for notifying Article 50.
He said: "The reason for that may be this. Nobody ever thought there was going to be a Leave vote."
Arguing that the prerogative should not be used, he said: "If the rule of law is to mean anything, even sovereigns must be constrained by it."
Helen Mountfield QC, appearing for the crowdfunded "People's Challenge" to the Government's Brexit strategy, urged the judges to uphold the High Court ruling.
Ms Mountfield said the people she represented "consider EU citizenship a fundamental part of their identity".
If they were to be deprived of the domestic law rights they enjoyed under EU law that should be done "by elected representatives in Parliament", she said.
That was important "in a democratic society which is based on the separation of powers (between the Government and Parliament) and the rule of law".
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