The Supreme Court on Thursday heard the final day of arguments in the Government’s fight for the power to trigger Article 50.
The case, before a record 11 Supreme Court justices, is due to finish today, but a decision is not expected to be released until January.
The court is being asked to overturn a High Court ruling that the Prime Minister must seek MPs’ approval to trigger the process of taking Britain out of the European Union.
In a decision that infuriated Brexiteers, three senior judges said Theresa May lacked power to use the royal prerogative to trigger Article 50 of the Lisbon Treaty and start the two-year process of negotiating Brexit without the prior authority of Parliament.
If the Supreme Court appeal is unsuccessful, and any potential further appeal to the European Court of Justice in Luxembourg also fails, the Government’s plans for Brexit could be thrown into disarray.
But May has made it clear she still intends to give an Article 50 notification by the end of next March to start the leave negotiations with 27 other EU countries.
The High Court ruling was won by Gina Miller, 51, an investment fund manager and philanthropist who was selected to bring the lead case.