Britain's highest court is on Monday to begin hearing the government's appeal against a ruling that it must first seek parliamentary approval before beginning the process to leave the European Union.
The landmark case will be heard by 11 judges.
What are the implications and likely government response to its ruling, expected in January?
Prime Minister Theresa May will be free to invoke Article 50 of the EU's Lisbon Treaty, the formal procedure for leaving the bloc, whenever she wants.
This would allow her to stick to her March timetable for triggering Brexit.
Should the Supreme Court uphold the original High Court ruling, the government is almost certain to immediately introduce a short bill to begin the Brexit process.
The bill would need to be approved by the House of Commons and the upper House of Lords, both of which could vote down the bill or seek to amend it by setting the terms of the divorce and thereby delay the process.
The government is reported to have prepared a three-line bill that it believes will be "bomb-proof" to amendments, and which could be approved within two weeks.
Despite being overwhelmingly in favour of remaining in the EU during the referendum campaign, MPs in the House of Commons are unlikely to vote down the bill for fear of stoking voter anger.
However, its passage through the unelected House of Lords -- where Theresa May's Conservative Party does not enjoy a majority -- is less certain, although any attempts to block the bill will outrage Brexit supporters.
The government could also appeal the case to the European Court of Justice, but this appears to be highly unlikely given the delays and further complications that could cause.
Parliament's refusal to pass the bill would be seismic for both domestic politics and for Britain's departure from the European Union.
With the odds heavily in her favour, May would be expected to trigger a general election, exposing Brexit-blocking MPs to the wrath of the public.
Such a result would likely be disastrous for the main Labour opposition, who are trailing in the polls, but also risks splitting May's Conservatives between Remainers and Leavers.
One of the 11 Supreme Court judges who will hear the appeal gave the government an extra headache when she suggested that EU legislation might have to be entirely replaced in British law before Brexit could begin, dashing May's hopes of rushing through a short bill.
Under current plans, the government will introduce the "Grand Repeal Bill", which will bring all EU laws onto the UK books as it leaves the union.
The government planned to introduce the bill after triggering Article 50, but such a ruling could force it to bring forward its passage, throwing the timetable into jeopardy.
- What about Scotland, Wales and Northern Ireland?
The picture is further complicated by the presence of the Scottish and Welsh governments, which are asking the judges to rule that their own devolved parliaments can have their own vote on triggering Article 50.
The Supreme Court will also be hearing an appeal calling for the Northern Ireland assembly have a vote, brought by Raymond McCord, a victims rights campaigner.
He is concerned that Brexit may result in Britain withdrawing from the European Convention of Human Rights, which he fears would undermine his fight for justice for his murdered son.
If the judges find in favour of these appeals, May's chances of invoking the article by March will almost certainly evaporate, and will raise the spectre of the devolved governments, particularly the EU-backing Scotland, attempting to block Brexit altogether.