The UK Supreme Court on Monday started hearing the UK Government’s appeal against a ruling by the High Court in October that insisted the prime minister must get parliamentary approval before triggering the process by which the UK starts its withdrawal from membership of the European Union (EU).

Central to the issue is Article 50 of the 2007 Lisbon Treaty on the European Union, which permits any member state to notify the European Council of its decision to withdraw. Once notification is given, the member state, the UK in this case, will cease to be a member of the European Union either on the date of entry into force of a withdrawal agreement, or two years after the notice is given, unless that period is extended by agreement between the notifying member state and the remaining members of the European Council. The agreement for withdrawal needs the consent of the European Parliament and the approval of at least 20 countries with 65% of the EU population in the European Council acting by a qualified majority.

UK Prime Minister Theresa May announced her intention to serve the Article 50 notice without first consulting Parliament on the terms on which her government would negotiate the UK’s withdrawal and future relationship with the EU. The government is relying on the Queen’s prerogative powers, which derive from the monarch’s historical position as an all-powerful ruler. However, those powers have been limited by legislation beginning with the Bill of Rights in 1689, which cemented the monarch’s subservience to Parliament.

The challenge was brought by a UK investment manager, and a hairdresser who explained his decision to challenge the government’s intention to The Guardian newspaper in the following terms: “I have never challenged the result of the referendum — in fact I voted for Brexit in the referendum for the sole reason that I wanted power to be returned from Europe to the British parliament. But I did not think it was right for government then just to bypass parliament and try to take away my legal rights without consulting parliament first.”

The UK voted to leave the EU by a slim margin of 51.9% to 48.1%. The effect of a rejection of the government’s appeal by a majority of the 11 Supreme Court judges sitting together for the first time in the court’s history is not to deny the fulfillment of the UK public’s vote for Brexit, but may undermine the government’s strategy for negotiating its withdrawal.

The opposition Labour party has said it will try to amend an Article 50 bill — if one is required — to ensure single market access and workers’ rights are protected. EU leaders have already made clear that access to the single market is not possible without respecting the EU principle of free movement of people, something that convinced many British voters to vote leave the EU in June, as control over the UK’s borders and immigration was a key part of the campaign to leave the EU.

As I wrote in July, there are a number of examples that already exist to point to the sort of relationship the UK might negotiate with the EU after withdrawal, but the government has so far been careful to play its cards close to its chest. The government fears that a debate in Parliament over Article 50 will involve showing its hand before it gets to Europe and place the UK at a disadvantage in the negotiations for withdrawal.

So the stakes are high and all eyes will be on the Supreme Court over the coming weeks. The appeal before the Supreme Court is scheduled to last until Thursday, with a judgement expected in January 2017.