The UK's biggest constitutional case in decades - on how the government can begin the process of the UK leaving the European Union - is being decided at the Supreme Court. What is it all about? The UK's highest court is determining how the government can trigger Article 50 of the Lisbon Treaty - the document which sets out the constitutional basis of the European Union - in order to leave. The government says it can do so without an act of Parliament, using executive powers. Its opponents in court say it cannot.
Brexit case to begin at Supreme Court
What next if ministers lose?
If the government loses and has to introduce a bill in Parliament, both its timetable for Brexit and its desire to keep its plans under wraps could be blown off course.
The case is not about the merits of whether the UK should leave the EU. That was decided in the referendum in June. It is about determining the lawful process under the British constitution for leaving.
On 3 November, three senior High Court judges, including the Lord Chief Justice Lord Thomas, head of the judiciary, ruled that the government did not have the power to trigger Article 50 using what is known as "prerogative powers".