The Supreme Court is to rule on Tuesday on who has the power to invoke the UK's withdrawal from the EU.
What is the Supreme Court deciding?
Whether the Prime Minister or Parliament has the final say in deciding when, and if, Article 50 of the Lisbon Treaty should be invoked.
What is Article 50?
It is the formal mechanism which a nation state must trigger if it wishes to leave the European Union.
What does Article 50 say?
"A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.
"The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period."
Why is the Supreme Court ruling on this?
Because the Government appealed against a High Court ruling in November which blocked Prime Minister Theresa May using the royal prerogative to launch Britain's exit from the EU without Parliament having a say.
What happens next?
If the Supreme Court rules in favour of the Government, Mrs May has stated she intends to trigger Article 50 by the end of March. If it upholds the High Court judgment, Parliament will debate and vote on invoking the measure.
:: Was the result of the June referendum binding on Parliament?
No. It was a consultative plebiscite, though few people on either side mentioned that at the time.
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