One of the most important constitutional cases in British legal history will sit for its second day on Tuesday.
The Supreme Court is hearing an appeal from the Government to overturn a High Court ruling that said the prime minister could not use the royal prerogative to trigger Article 50 and begin the formal Brexit process.
On the first day of evidence, the 11 justices were told by Attorney General Jeremy Wright that triggering Article 50 was the "logical conclusion of a process in which Parliament has been fully and consciously involved".
The Government's top law officer said that process was one in which Parliament had "resolved to put a clear and decisive question about our nation's future to the British people, and in which Parliament expected the Government to act on the answer they gave".
He added: "None of this means, of course, that Parliament will not be closely involved in the process of the UK's withdrawal from the EU over the coming months and years."
Senior government lawyer Mr James Eadie also told the court that the government's use of prerogative powers as a "a long-standing, well-recognised set of powers firmly established in our constitutional arrangements" were "fundamental to our constitution and essential to effective government".
"What is the act in question here? It's the giving of Article 50 notice. And there's been no control over that at all," said Mr Eadie.
In a decision that infuriated Brexiteers, three judges said Theresa May requires the backing of Parliament to trigger Article 50.
Mr Wright told the justices that the case was of "great constitutional significance in which there is understandable and legitimate interest both inside and outside this courtroom".
He said the High Court had reached the "wrong" decision. It was for the Government to exercise prerogative powers in the conduct of the UK's affairs on the international plane.
The position of those who had brought the case and others had always been "that they have no interest in derailing Brexit but only in defending Parliament's role in the process".
Mr Wright continued: "But if this is all about standing up for Parliament, I say Parliament can stand up for itself.
"When it comes to leaving the European Union, Parliament has had full capacity and multiple opportunities to restrict the executive's ordinary ability to begin the Article 50 process and it has not chosen to do so."
If the historic appeal is unsuccessful, and any potential further appeal to the European Court of Justice in Luxembourg also fails, the Government's plans for Brexit could be thrown into disarray.
But Mrs May has made it clear she still intends to give an Article 50 notification by the end of next March to start the leave negotiations with 27 other EU countries.
The Government argues that the panel of High Court judges erred over Article 50 and its use was legally justified by the June 23 referendum vote in favour of quitting the EU.
The High Court ruling was won by Gina Miller, 51, an investment fund manager and philanthropist who was selected to bring the lead case.
Her case is being supported by "concerned citizens" drawn from all walks of life, including London hairdresser Deir Dos Santos, 37, who helped start the legal battle over Brexit but, say his lawyers, has been forced underground after receiving "vile" hate mail.
Supreme Court President, Lord Neuberger, warned the public against threatening or abusive behaviour during the first day of the hearing.
The Scottish and Welsh governments and the Attorney General for Northern Ireland are all intervening in the Supreme Court case. A ruling will not be given until the new year.