Judges will tomorrow give their verdict on whether Theresa May must consult gain MPs consent to trigger Article 50 and begin the process of exiting the European Union.
Campaigners argue that May cannot do so without the support of the House of Commons, and argue that the summer's European referendum alone does not confer power to quit the EU.
While lawyers for the government have said that some parliamentary approvals will be necessary for Brexit, it maintains that activating Article 50 remains the prerogative of Downing Street.
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Judges are expected to give a verdict at around 10am tomorrow morning, with both sides expected to seek a Supreme Court appeal if they lose.
Theresa May has promised to begin the process by the end of March next year, with the UK then facing up to two years of talks to establish a new relationship with the EU.
That window can then be extended with the approval of both the UK and all 27 remaining EU member states.
It comes after months of parliamentary debate on the role of MPs to scrutinise Brexit plans.
Theresa May recently revealed that MPs would be offered a series of high-level debates on quitting the EU, but declined to go as far as guaranteeing a vote on the process.