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Lord Pannick QC.
The verdict is set to be handed down at 09:30 a.m. (GMT) by Supreme Court President Lord Neuberger, to a packed audience of lawyers, journalists, and members of the public who arrive at the Wesminster court early enough to secure a seat for the ruling.
The verdict on Tuesday will bring to an end a legal battle that began in the summer. Gina Miller, a London-based investment manager, launched a legal case arguing that it was unlawful for the prime minister to invoke Article 50 of the Lisbon Treaty without first securing the approval of Parliament.
Miller, represented by Lord Pannick QC, argues that triggering Article 50 without first consulting MPs would mean rights enjoyed by Brits as EU citizens put into law by Parliament being removed without Parliament's permission.She explained her case in greater depth
She explained her case in greater depth when she spoke to Business Insider in August.
How we got this far
Her case - along with those of other claimants - was heard in the High Court in November. Three of the country's most senior judges sided with Lord Pannick and other claimants, ruling that the prime minister must secure parliamentary approval before initiating Britain's formal divorce from the 28-nation bloc. The government opted to appeal this decision, however, meaning it had to be referred the Supreme Court, the most powerful court in the land.
Attorney General Jeremy Wright QC and James Eadie QC kicked off the government's appeal by arguing that MPs had a number of opportunities to limit the scope of the royal prerogative powers enjoyed by the prime minister prior to June's EU referendum taking place. Lord Pannick responded by reiterating the underlying argument that triggering Article 50 without first consulting parliament would lead to rights which parliament established, and only parliament can legislate on, being nullified. Read a summary of Supreme Court proceedings here.
If, as expected, the Supreme Court upholds the High Court's decision, then the triggering of Article 50 will officially become a parliamentary matter. The government will likely pass an act of parliament at first attempt, but May's plan to lead Britain out of the single market have riled a number of MPs across the house, with Labour, SNP, Liberal Democrat, and Green MPs threatening to block the legislation if the government refuses to make concessions.
While this case comes to an end on Tuesday, a new Brexit legal battle will commence on Friday, when the Irish Court will begin to deliberate whether Article 50 can be revoked once triggered. The case was launched by barrister Jo Maugham, who told Business Insider that if the Article 50 process can be reversed then it would give the public the chance to decline a Brexit deal. Maugham also believes there is evidence to suggest that Article 50 has already been triggered.
Business Insider will be reporting live from the Supreme Court on Tuesday morning.