The 11 Supreme Court judges are expected to rule against the govt. in a move which could delay PM Theresa May triggering EU's Article 50.
The 11 Supreme Court judges are expected to rule against the government in a move which could delay Prime Minister Theresa May triggering Article 50 of the EU's Lisbon Treaty, which would formally begin exit negotiations.
Tuesday's decision follows a High Court ruling against the government in November, in a case which attracted protests and abuse against the lead claimant.
Businesswoman Gina Miller said she has suffered death threats and racist taunts since bringing the case.
"Things that were considered unacceptable are now acceptable," she told AFP ahead of the Supreme Court decision.
Anger was also directed at the High Court judges following their decision, being branded "Enemies of the People" by one newspaper.
The legal challenge has tapped into divisions within British society after the June referendum which saw 52 percent vote to leave the EU.
Brexiteers have claimed the case is an attempt to block Britain's departure from the European Union, but MPs are not expected to vote against triggering Article 50.
If they lose the case ministers are preparing to rush emergency legislation through the Houses of Commons and Lords -- and the opposition Labour party has promised not to block it.
"Even if the Supreme Court rules that May cannot bypass parliament in Westminster, parliamentarians will be unlikely to go against the vote of the British people and block Brexit," said Agata Gostynska-Jakubowska, a research fellow at the Centre for European Reform.
Labour party leader Jeremy Corbyn has however said he will take the chance to table amendments to the bill, including maintaining access to Europe's single market.
Such a move would go against the premier's plan for a "hard Brexit" announced last week, through which May said Britain will leave the single market in order to control EU immigration.
Opposition amendments could delay May's timetable to trigger Brexit talks by the end of March, but the Supreme Court could further scupper her plans.
Judges are also set to rule on whether the devolved governments in Scotland, Wales and Northern Ireland must have a say on the prime minister's plan.
Gostynska-Jakubowska said May could find it difficult to trigger Article 50 by her March deadline if the judges demand greater involvement from the devolved nations, while deeming such an outcome unlikely.
While Wales voted for Brexit, a majority in Northern Ireland and Scotland voted to remain in the EU.
Northern Ireland is facing its own political turmoil with snap elections on March 2 after the power-sharing executive in Belfast collapsed earlier this month.
The pro-EU Scottish National Party, meanwhile, has 54 MPs in the 650-seat House of Commons and also controls the government in Edinburgh.
Kathleen Brooks, an analyst at City Index Direct, said giving greater power to the devolved governments would be the "worst outcome" for May at the Supreme Court.
"This outcome would likely delay the UK's Brexit plans, and, trigger the break-up of the UK, especially if it leads to another independence referendum in Scotland," she said.