Britain's most senior judges will rule today on whether Theresa May has the absolute power to trigger Article 50 without a parliamentary vote.
All 11 Supreme Court justices have sat as a panel to hear the appeal. This is the endgame for a legal challenge to the Government's right to start negotiations, following a landmark High Court ruling last November whereupon the Prime Minister was ruled to lack the authority to initiate Article 50 without consulting MPs.
If the ruling is in the Government's favour, Mrs. May will start negotiations by the end of March. If it upholds the High Court judgment, Parliament will have to debate and vote on invoking Article 50.
Although the Government is highly unlikely to lose a parliamentary vote on Brexit, the progress of the legislation through Parliament could be frustrated.
Labour leader Jeremy Corbyn has said his MPs won't block Article 50, but hasn't said if he'd impose a three-line whip to make them vote for the measure.
Lib Dem leader Tim Farron has called on Labour to clarify its position on Brexit, and said his own party will vote against the triggering of Article 50 unless there is a second referendum on the deal Theresa May negotiates with Brussels.
Farron said: "There's nothing progressive about giving Theresa May a blank cheque to decide what Brexit she thinks is best.