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LONDON (Reuters) - The British government's appeal against a legal ruling that it needs parliamentary approval to trigger the formal process of leaving the European Union is being heard in the country's top court this week.
Prime Minister Theresa May has announced that she will invoke "Article 50" of the EU's Lisbon Treaty by the end of March and begin the formal Brexit negotiations with Brussels.
But London's High Court ruled last month that the government does not have the constitutional right to start the process without the backing of lawmakers.
How does the Supreme Court work and how will the case be argued?
The Supreme Court is the highest judicial body in Britain. It is the final court of appeal for all UK civil cases, and criminal cases from England, Wales and Northern Ireland, focusing on cases of political and constitutional importance.
One of its most high-profile rulings was in 2014 - on whether the 1961 Suicide Act, which banned assisted suicide, should be deemed unlawful. The court ruled the issue was a moral matter for parliament and not judges.
Last year it dismissed an appeal by Donald Trump - now U.S. president-elect - to try to stop wind farms being built near his Scottish golf resort. It also threw out the government's attempts to prevent private letters from heir-to-the-throne Prince Charles to ministers from being made public.
The Supreme Court currently has 11 justices who are selected by an independent commission and must have either been a High Court judge for two years or a practising lawyer for 15 years.
The president of the court is David Neuberger, who worked at merchant bank N M Rothschild & Sons before switching to law in 1974. The vice-president is Brenda Hale, who was Britain's first female law lord.
The other justices are Jonathan Mance, Brian Kerr, Anthony Clarke, Nicholas Wilson, Jonathan Sumption, Robert Reed, Robert Carnwath, Anthony Hughes and Patrick Hodge.
Usually four or five justices sit on a panel considering cases. However for the first time, all 11 justices are sitting "en banc" to hear the Article 50 challenge.
A simple majority is needed for a judgment.
The case will be held over four days from Dec. 5-8. The judgment will follow at a later date, probably in mid to late January.
Unlike other British courts, the Supreme Court's judges do not wear wigs nor do they sit in an elevated position. While TV cameras are barred from nearly all other courts, its hearings and judgments are broadcast live on its website.
It centres on who has the constitutional right to invoke Article 50 of the EU's 2009 Lisbon Treaty, the formal process by which Britain notifies the European Union of its intention to leave the bloc, kicking off two years of negotiations.
All parties in this case agree the Article 50 process is irrevocable, so as soon as it is triggered, Britain will inevitably leave the EU at some stage. However, some EU and legal experts believe this is a misreading of Article 50 and that Britain could change its mind at some time in the future even after triggering it.
It is possible that the justices might themselves seek an answer to this question which would mean they would have to consult the European Court of Justice, something which might delay the case.
At the High Court, the claimants successfully argued only parliament could start the exit process. Their case was that rights that were incorporated into British domestic law by the 1972 European Communities Act by which Britain joined the bloc could not be taken away without parliament's approval as it is the sovereign body in Britain's unwritten constitution.
Furthermore, they say the referendum was only advisory and gave no indication about when and on what terms Britain should leave, and those issues should be a matter for parliament
The government argues that it can invoke Article 50 without lawmakers' approval, using a historical power known as "royal prerogative" where ministers act on behalf of the monarch.
It says it is established constitutional convention for ministers to use royal prerogative when making or withdrawing from international treaties. Furthermore, the government argues that parliament approved the referendum on the understanding its outcome would be enforced.
The lead claimant in the successful High Court challenge was investment fund manager Gina Miller, who has since received racist and sexist intimidation. Hairdresser Deir Tozetti Dos Santos is the second claimant.
Other "interested parties" involved in the claim include "The People's Challenge", featuring an Englishman of Bangladeshi origin, an Irishman, two Scotsmen resident in France, a Welshman and a Gibraltarian, whose wife is Spanish, and British children of EU nationals.
Those who will be allowed to put forward legal arguments include lawyers for the Scottish and Welsh devolved governments and the Independent Workers Union of Great Britain, whose members are mostly low-paid migrant workers in London.
The government case is formally being fought on behalf of the Secretary of State for Exiting the European Union, David Davis. The Attorney General Jeremy Wright, the government's top lawyer, led arguments for the government in the High Court and reprised that role in the Supreme Court.
Since May announced she would invoke Article 50 by the end of March, sterling has fallen against the dollar, hitting its lowest levels in over three decades at one point. Jittery markets fear her government is heading for a "hard Brexit", prioritising curbing immigration over remaining in the EU single market.
Investors believe the greater the involvement of lawmakers, most of whom supported staying in the EU, the greater the chance of a "soft Brexit" where Britain prioritises securing some single market access and close ties with the bloc.
The outcome of the appeal could also have an effect on the Brexit timetable.
If the government loses it will have to secure some form of parliamentary approval to trigger Article 50.
This could be achieved through a substantive motion - a proposal put forward for debate and a vote - which would take little time. However, the claimants say there needs to be new primary legislation that passes through both parliamentary chambers, a far more complicated process.
While it is possible legislation could be introduced and passed between the time of a final judgment late this year, and May's end-of-March 2017 deadline, it is likely to be tight and may result in the triggering being pushed back.
The 1972 European Communities bill, which set the terms of Britain's entry into the European club, involved a total of about 40 days of debate during its passage through parliament, according to the Institute for Government.
Lawmakers could also add amendments demanding additional scrutiny of the government Brexit plans which could complicate their negotiating position.
If the government wins the appeal, then May will be able to press ahead with her plans to invoke Article 50 by the end of March.
Lawyers for the claimants say the losing side could still conceivably appeal to the European Court of Justice but that this is highly unlikely. Miller has indicated she would not pursue the case in Europe, while it would be politically embarrassing, if not impossible, for May to seek to have the European Court overturn the verdict of the British court.
Lawmakers in the lower house, the House of Commons, are unlikely to try to block Brexit, as it was backed in a popular vote, and a Reuters survey suggested many MPs who voted to "remain" would now approve the triggering of Article 50 in a parliamentary vote.
However, a cross-party group of lawmakers, who support a "soft Brexit" have demanded a greater say for parliament in negotiations and say they might try to pass amendments that guarantee this.
The opposition Labour Party has said it would seek amendments to ensure access to the European single market and to protect workers' rights.
Should the government need to pass legislation, the government may also face trouble in the House of Lords, where the ruling Conservatives do not have a majority. If the Lords were to block the bill the government could decide to overrule it using the Parliament Act, although it cannot re-table the bill until the next parliamentary session.
The new parliamentary session usually starts in May or June, so that would delay the Brexit process significantly.
Editing by Guy Faulconbridge and Ralph Boulton