Judges began hearing three days of highly-charged arguments over whether it was lawful for UK Prime Minister Boris Johnson to advise Queen Elizabeth II to prorogue, or suspend, parliament for more than a month, as the clock ticks down to Britain's October 31 EU exit date.
Campaigners challenging the suspension argue that Prime Minister Johnson's motivation for shuttering the chamber from last week to October 14 was to avoid MPs trying to stymie his plans for Britain to leave the European Union with or without a divorce deal from Brussels at the end of next month.
But Prime Minister Johnson's lawyers insisted it was not a matter for the courts to get involved in, and said that in any case, it was permissible to suspend parliament for overtly political reasons.
Richard Keen, the government's top Scottish legal adviser, told the Supreme Court of past cases where the executive had prorogued parliament to avoid scrutiny and force through its programme, which Prime Minister Johnson has always insisted was not his reasoning.
Mr Keen said the prime minister would take all necessary steps to comply with whatever the court decided.
But when he could not say in court that the Prime Minister would not simply prorogue parliament again, one of the judges asked for a written response on what measures Mr Johnson would take.
Richard Keen, the government's top Scottish legal adviser, told the Supreme Court of past cases where the executive had prorogued parliament to avoid scrutiny. Source: UK SUPREME COURT
Crunch time
The UK's narrow 2016 referendum decision to leave the EU is coming to a head.
The politically-charged court case, unprecedented in Britain, could lead to parliament being recalled and Prime Minister Johnson's political hand being severely weakened in the run-up to October 31.
A defeat in court for Mr Johnson would leave him open to charges that he effectively lied to Queen Elizabeth and likely trigger calls for his resignation.
In a sign of the unease still consuming the country, demonstrators wanting Britain to remain inside the EU traded barbs with pro-Leave rivals outside the central London court, which sits directly opposite the shuttered parliament.
The Supreme Court has called in the maximum 11 of its 12 judges to hear the case, so that there cannot be a tied decision.
They must rule on whether they even have the right to adjudicate on the politically contentious issue, before considering Prime Minister Johnson's motivations for the move.
In lower court decisions, the High Court in England said it was not a matter for the courts to decide upon, while Scotland's highest civil court called the suspension "unlawful".
An anti-Brexit demonstrator protests outside The Supreme Court in London. Source: AP
Lawyer David Pannick, representing campaigners appealing against the High Court decision, said Mr Johnson had asked the monarch to suspend parliament "to avoid what he saw as the risk that parliament, during that period, would take action to frustrate or to damage the policies of his government."
He said the Prime Minister had perpetrated "unlawful abuse of power" and the long prorogation was "strong evidence that the prime minister's motive was to silence parliament".
But Mr Keen insisted that only seven sitting days had been lost rather than five weeks, as MPs are off at their party conferences.
Parliament could be prorogued for "naked political" reasons, he said, adding that any ruling against the government would need to explain what an unlawful political reason would be.
Supreme Court President Brenda Hale said the hearings "will not determine when and how the UK leaves the EU". The court has not said when it expects to reach a decision.
Cabinet talks
Prime Minister Johnson chaired a cabinet meeting on Tuesday, the day after holding his first face-to-face talks with European Commission chief Jean-Claude Juncker and EU Brexit negotiator Michel Barnier.
Following the meeting in Luxembourg, he insisted there remained a "good chance" of striking a new divorce deal, if there was "movement" from the bloc.
But Mr Juncker's office appeared more pessimistic, saying Britain was yet to come forward with "legally operational solutions" to problems with the existing withdrawal agreement struck by Prime Minister Johnson's predecessor Theresa May.
Prime Minister Boris Johnson arriving to meet Prime Minister Xavier Bettel at the prime ministers office in Luxembourg. Source: Press Association
Ahead of its suspension, parliament rushed to pass a law forcing Mr Johnson to ask Brussels for a delay if no compromise emerges from an October 17-18 EU summit.
Prime Minister Johnson is hoping widespread fears of a chaotic "no-deal" end to Britain's 46-year involvement in the European project will push its leaders to compromise.
