Factbox: The legal challenge over who can trigger Britain's exit talks from EU

By Michael Holden and William James (Reuters) - The UK Supreme Court will rule on Tuesday whether British Prime Minister Theresa May can trigger Britain's exit from the European Union without seeking prior approval from parliament. May has said she intends to invoke Article 50 of the EU's 2009 Lisbon Treaty by the end of March, beginning a two-year divorce process. However, London's High Court ruled in November that she could not start the formal exit process without parliament's assent, prompting ministers to appeal to the Supreme Court, the highest judicial body in the United Kingdom. It will give its ruling at 0930 GMT on Tuesday. For news on Brexit, click on BRXT For a weekly round-up of Brexit news: http://share.thomsonreuters.com/assets/newsletters/Specialpackages/BREXIT_Weekly.pdf WHAT'S IT ALL ABOUT? The case is simply about who has the right to invoke Article 50, the process means by which Britain notifies the European Union of its intention to leave the bloc and begins two-years of negotiations. May says the government can take the action without lawmakers' approval, using a historical power known as "royal prerogative" where ministers act on behalf of the monarch. The claimants argued this would be unconstitutional and that only parliament can agree to leaving the EU because to do so would strip Britons of rights granted by parliament. They want the government to introduce legislation that lawmakers can debate and vote on. WHO'S INVOLVED? 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 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; another group of British expatriates; and British children of EU nationals and non-EU carers of British children or the disabled. They were joined at the Supreme Court by lawyers representing the Scottish and Welsh governments and politicians and human rights groups from Northern Ireland. The UK government's case was being put on behalf of the Secretary of State for Exiting the European Union, David Davis. For the first time in the court's seven-year history, all 11 justices sat on the panel hearing the case. WHAT WILL HAPPEN ON TUESDAY The President of the Supreme Court David Neuberger will deliver the ruling at 9:30 a.m. (0930 GMT) on Tuesday in a statement that will last for little more than five minutes. For news on Brexit, click on [BRXT] WHY IS IT SIGNIFICANT? If the government loses its appeal, parliament will have to vote on whether Article 50 should be triggered. A clear majority of lawmakers backed staying in the EU ahead of June's referendum when Britons voted by 52-48 percent to leave. Some investors believe that the involvement of lawmakers will increase the chance of a "soft Brexit" where Britain prioritizes access to the European single market and close ties with the bloc over immigration controls. Markets rallied during a speech by May last week when she said Britain would quit the single market but parliament would be given a vote on the final deal. COULD PARLIAMENT BLOCK BREXIT? If the Supreme Court rules in the government's favor, May can continue with her planned timetable. If she loses, the government will have to secure some form of parliamentary approval to trigger Article 50. Ministers have said this would mean legislation which would have to be passed by both parliamentary chambers, the House of Commons and House of Lords. Sources from May's Conservative Party and the main opposition Labour Party say a bill could be rushed through to ensure it is passed before the end of March. Although a majority of Members of Parliament (MPs) in the Commons supported staying in the EU, most would now vote to trigger Article 50, according to Reuters research. Labour leader Jeremy Corbyn has said his MPs would be instructed to vote to invoke Article 50, although The Times newspaper has suggested some 60-80 might refuse. Even with the support of the pro-EU Scottish Nationalists and Liberal Democrats, that is unlikely to be anywhere enough to stop Brexit in the 650-seat Commons. The government may face trouble in the House of Lords, where the 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 - which it would need to do to get final approval in the Commons - until the next parliamentary session. The new parliamentary session usually starts in May or June, so that would delay the Brexit process significantly. However, unelected peers in the Lords are unlikely to vote against something backed by both the Commons and the public in a referendum. BREXIT AMENDED? In a speech on Jan. 17, May laid out her plans for Brexit, with clean break from the EU and a 12-point priority list which included leaving the single market and seeking global free trade deals. While lawmakers might not try to block Article 50 being invoked, they may well seek to amend the law that authorizes it being triggered, to put demands on May such as ensuring the greatest possible access to the single market and additional scrutiny of ongoing negotiations. Labour's Corbyn has said he would not block the bill but his aides say he would try to amend it to include a vote on May's negotiating plan. IS IT THE END OF THE LEGAL CHALLENGES? No, two other challenges are already being prepared. One case is being taken to London's High Court, arguing that even if Britain leaves the EU, it would not leave the European Economic Area, which includes EU member states as well as Norway, Iceland and Liechtenstein and provides access to the single market and its free movement of goods, capital, services and people. The second challenge is potentially more significant. This seeks to ascertain whether Article 50 can be reversed after it has been revoked. The British government says it cannot, some European leaders are not so sure. That case is due to be launched at the High Court in Dublin this month. If ultimately the European Court of Justice decided Article 50 could be reversed, it would allow the possibility of Britain staying in the EU if the two-year talks are inconclusive, or even allow for a second referendum. (Editing by Guy Faulconbridge and Robin Pomeroy)