The Supreme Court will begin a landmark legal hearing today, Monday, into whether Parliament's consent is required before official Brexit negotiations can begin.
Its 11 justices will hear a government appeal against last month's High Court ruling that only Parliament has the authority to trigger Article 50.
The hearing, to be streamed live by the BBC,
is expected to last four days, but the verdict is not due until next year.
The outcome will have implications for Theresa May's strategy for EU exit.
The UK voted to leave the EU, by a margin of 51.9% to 48.1%, in a referendum in June.
The prime minister has said she intends to officially notify the rest of the EU of the UK's intention to leave - beginning two years of talks over the terms of separation - by the end of March.
But campaigners, led by investment manager Gina Miller and hairdresser Deir Dos Santos, insist that decision can only be taken by Parliament.
The High Court ruled in Ms Miller's favour, arguing that rights conferred by Parliament when it passed the 1972 European Communities Act - which paved the way for the UK to join the then European Economic Community - were likely to be affected by Brexit.
As a result, it concluded, any process leading to the potential withdrawal of rights could only be determined by Parliament.
In its judgement, the High Court ruled "the powerful constitutional principle that the Crown has no power to alter the law of the land by use of its prerogative powers is the product of an especially strong constitutional tradition in the UK".
The ruling led to fierce criticism from some elements of the press and Brexit-supporting MPs, but in her submission ahead of Monday's hearing, Ms Miller said the Court had a "duty to decide questions of law".
Ministers will have a number of options if they lose the appeal, but it has been reported that a 16-word bill is being prepared which could be fast-tracked though Parliament, asking MPs and peers "to give permission" to the government to trigger Article 50 in time to meet the March deadline.
The respective legal teams will be led, on the government side, by Attorney General Jeremy Wright while crossbench peer Lord Pannick will head the team acting for Ms Miller.
In a statement on its website, the justices said they were "aware of the public interest in this case and the strong feelings associated with the wider political questions of the UK's departure from the EU, which we stress are not the subject of this appeal".
The case is one of the most keenly anticipated of its kind in recent history. Additional seating has been provided to allow 115 members of the public to watch proceedings while more than 80 journalists from around the world have been accredited to cover the event.
The court will also consider two separate but connected legal challenges to the exercise of prerogative powers brought by Northern Irish campaigners.
And in another twist, the Scottish and Welsh Governments have been granted permission to intervene in the case to establish matters of important legal and constitutional principle, including the basis on which the Article 50 process might need to be sanctioned by their devolved legislatures.
Other interested parties and intervening groups - who will be making oral arguments before the court - include citizens of EEA countries living in the UK known as AB parties and Grahame Pigney's crowd-funded People's Challenge.
The court will reserve its judgement to a later date, widely expected to be in January.