In early November, the High Court had rejected government arguments that clauses in a piece of legislation from 1972 gave it powers to act without parliamentary approval when it came to triggering Article 50, which gives European Union member states a two-year period to withdraw.
The Government's use of the royal prerogative to trigger the process of taking Britain out of the European Union is not being done on a "whim" or "out of a clear blue sky", the UK's highest court has heard.
By the end of the day, however, justices of the Court were clearly questioning this assertion and legal experts last night were commenting that the Government's case was coming under robust questioning.
He goes on to say that the hypothetical scenario of Greece leaving the European Union would affect United Kingdom citizen's EU rights. He insisted that the case was to do with legal issues and judges would decide the case according to law.
He said that parliament will have already been "fully and consciously involved" when the government triggers Article 50, and that the government's use of the Royal Prerogative will therefore be lawful.
The government said the powers were not a "relic" but a key part of the constitution and that parliament could have chosen to restrict ministers power to act but had chosen not to.
The decision enraged Brexit supporters and some newspapers who accused judges of thwarting the will of the 52 percent who voted "Leave" in a June 23 referendum.
The case was initially brought by Gina Miller, an investment manager, and Deir Dos Santos, a London hairdresser, who both demanded a vote from lawmakers before implementing the non-binding referendum.
It is certainly the biggest case the Supreme Court has had to handle. This, Eadie suggests, means the power to trigger Article 50 should remain with the Government.
It was a "clear question" said Wright, "that raises issues going to the very heart of our constitutional settlement".
Even if the government loses, it is unlikely to stop Britain leaving the EU.
Police were forced to intervene as two men got into a scrap on the first day of the court hearing on whether the Government needs a vote from MPs before starting Brexit.
"The arguments this time round are pretty much the same as in the last case - and I suspect the outcome will be the same", said Nick Barber, associate professor of constitutional law at Oxford University.
The UK's most senior judge hit back at accusations that justices are straying into politics as he opened the historic hearing yesterday on whether parliament must approve Brexit.
Their lawyers successfully argued at the High Court that Britons would inevitably lose rights granted by parliament when leaving the European Union, and that under Britain's unwritten constitution such rights could only be taken away with lawmakers' approval.
He said: "The pro-parliamentary legal argument is that Article 50 affects rights derived from statute which the Government can not change unilaterally". We say, respectfully, that the divisional court was wrong about that.
The court will also consider interventions from the Scottish and Welsh governments and two cases from Northern Ireland, one brought by a group of politicians and civil society groups, and the other by Raymond McCord, a campaigner on behalf of victims of violence.
If the court upholds the decision, it would prove problematic for Prime Minister Theresa May, who has said that the government wants to have to trigger the exit clause by the end of March. The hearings will run till Thursday, with the justices expected to give their verdict in the New Year.