The Government's appeal against the High Court's ruling MPs must have a say before Brexit is triggered has begun in the Supreme Court.
The highest court in the land will rule whether or not Prime Minister Theresa May can invoke Article 50 to leave the EU without parliamentary approval.
Here is what you need to know about the hearing:
- What is Article 50 and what does it mean for the UK?
Article 50 forms part of the Lisbon Treaty which was signed by all members of the EU in December 2007.
It sets out the rules for the process of a member state leaving the union and starts a two-year negotiation period.
- The legal challenge
The judgment came after legal proceedings brought by businesswoman Gina Miller, along hairdresser Deir dos Santos, who argued the Government should not be able to bypass parliament.
Mrs Miller said: "I've never challenged the result of the referendum, I in fact voted for Brexit... for the sole reason that I wanted power to be returned from Europe to the British parliament.
"I did not think it was right for the Government than just to bypass Parliament and to try and take away my legal rights without consulting Parliament first."
Theresa May has previously suggested the clear referendum result and prime ministerial powers mean Article 50 could be triggered without consent of parliament.
- The High Court judgment
The Lord Chief Justice, Lord Thomas, said to do so would be contrary to "the fundamental constitutional principles of the sovereignty of Parliament".
The court rejected the Government's argument that prerogative powers were a legitimate way to give effect "to the will of the people" who voted by a clear majority to leave the EU in the June referendum.
- What was the Government's reaction to the ruling?
A Government spokesman quickly announced it would appeal against the decision to the Supreme Court.
A spokesman for the Department for the Exit of the European Union said: "We will robustly defend our position in the forthcoming appeal. As the Prime Minister made clear our work is on track and we remain committed to triggering Article 50 by the end of March next year."
- Who will hear the Supreme Court appeal?
All 11 Supreme Court justices will sit on the appeal panel, chaired by its President Lord Neuberger, to hear the case.
The hearing is expected to last four days and judgment will be reserved until a later day, likely to be in the New Year.
- Scottish and Welsh governments to pitch in to court battle
The Supreme Court judgment ruled both the Scottish and Welsh governments will be able to submit their legal objections to Mrs May triggering Article 50 without the consent of Parliament.
Scotland's Lord Advocate James Wolffe QC will argue that the consent of Holyrood should also be required.
In a written submission to the Supreme Court, Counsel General for Wales Mick Antoniw stressed he was "not seeking to reverse the referendum result" but said a new law would need to be passed at Westminster.
He also said Mrs May could not "short circuit" the requirement for the devolved institutions to give their consent.
The Supreme Court has also given permission for pro-Brexit group Lawyers for Britain to file written submissions on the case.
- Tory MPs urge Theresa May to drop appeal
Some senior Conservative MPs have urged the Prime Minister to abandon the appeal.
They include former head of the Government's Brexit preparations Oliver Letwin and former attorney general Dominic Grieve.
The pro-Remain Conservatives instead want the withdrawal process started as soon as possible with a Bill in Parliament.
- What happens if the Government loses?
The Government is reportedly considering alternative measures in the event its appeal fails.
These are said to include the drawing up of a draft contingency bill to keep the Prime Minister's timetable for triggering Article 50 on track.
It would mean MPs and peers will be able to influence the start of the Brexit process as they would have the chance to amend the bill.
A Government spokesman has previously refused to confirm the existence of such a bill.
It could also mean Mrs May will not be able to invoke the EU get out clause by the end of March and increase the likelihood of an early election.
- May faces challenging times even if the appeal is upheld
A number of MPs have warned they will vote against triggering Article 50 in Parliament unless major concessions are made.
Labour leader Jeremy Corbyn has said the party will only let Theresa May trigger Article 50 if she guarantees access to the single market.
Tim Farron, Liberal Democrat leader, has also said his party will vote against Article 50 unless there is a guarantee the final Brexit deal is put to a fresh referendum.
However, Brexit Secretary David Davis has insisted there will be no second referendum and accused opposition parties of trying to "wreck" Brexit negotiations.