Britain's Supreme Court will rule on Tuesday on whether the prime minister or Parliament has the right to trigger the process of taking Britain out of the European Union.
The 11 justices will either uphold an earlier ruling giving Parliament a direct role in invoking Article 50 of the EU's Lisbon Treaty or reject that ruling in favour of the government's claim it can do so without a vote in Parliament.
Article 50, which has never been used before, starts the formal process of taking Britain out of the 28-nation EU.
The legal battle follows a successful High Court action led by businesswoman Gina Miller.
A reversal of the lower court ruling would give Prime Minister Theresa May the authority to start Article 50 negotiations when she chooses without input from Parliament.
She has said, however, that Parliament will be asked to approve any Brexit deal reached at the conclusion of the talks.