The Supreme Court in Britain is to deliver its long awaited judgment on whether ministers or parliament there have the power to start the Brexit process.
In what is being seen as the most significant constitutional case in decades, 11 justices will decide if Prime Minister Theresa May can use inherited prerogative powers to trigger Article 50 - the formal mechanism for leaving the EU - without consulting the parliament.
The UK government is believed to be anticipating losing its appeal against an earlier High Court ruling, which said MPs should be involved.
James Arthur Hope, a former deputy president of the UK Supreme Court, said: "Well these sorts of cases come up once in your career.
"I think the justices will have been absolutely fascinated by it."
He stressed that judges are not there to assess the merits of Brexit.
"Judges are not in the business of politics at all, that is not their issue," he said.
"It is a fascinating issue of law...and when you get an issue of law you've got to have somebody who is competent to decide it and whose decision will command respect and that is what the decision in this case is all about."
That said, the judgment will be politically controversial and the government is believed to have drawn up different versions of legislation to put to parliament to anticipate any scenario the court could throw up.
The draft bills are believed to be short and to the point to try to ensure Mrs May can get permission to trigger Article 50 by her self-imposed end of March deadline.
"Fight for democracy"
Gina Miller, the businesswoman who was the lead claimant in the High Court case, is encouraged by the thought the Supreme Court will uphold the earlier ruling.
Insisting she is not attempting to thwart the result of the EU referendum, she says she simply wants MPs and peers to have a say as leaving the EU will lead to a loss of fundamental rights for UK citizens.
She said: "I am hoping that they (the Supreme Court justices) will uphold the rule of law when it comes to our parliamentary sovereignty and democracy, that no prime minister or executive can bypass parliament when it comes to people's individual rights."
Addressing the abuse and harassment she has suffered since bringing the case, Ms Miller still insists she is glad she brought the legal battle.
"Oh, it's absolutely been worth it to fight for democracy," she said.
The judgment will be handed down with the president of the Supreme Court David Neuberger reading out a summary of the justices' findings weeks after a four-day public hearing in early December.
They were provided with 30,000 pages of written argument and legal authorities to help them form an opinion.
But if their ruling goes the way expected, it is thought there will be anger in many quarters.
Former UKIP leader Nigel Farage accuses Ms Miller, and others who joined her campaign to have parliament involved, of ignoring the will of the people in the EU referendum.
All parties in the case have promised to respect the Supreme Court's judgment - but whatever happens, the British government is to be hit with other legal challenges over the European breakup in the coming weeks.
