The legal battle has major constitutional implications for the balance of power between the legislature and the executive. And has inflamed Britain's already raw wound over how and whether to leave the EU.
British government lawyers have argued that once the formal divorce talks on leaving the European Union are triggered there is no going back. But EU leaders have suggested Britain could still change its mind.
So who is right? The answer could have a significant impact on the course of Brexit and a court case being heard by the Supreme Court on Monday.
Prime Minister Theresa May has repeatedly said that "Brexit means Brexit" and that Britain will invoke Article 50 of the EU's Lisbon Treaty by the end of March.
But Article 50 does not say whether it can be revoked once it is invoked and the lack of clarity means that if lawyers ask for clarification the question would have to go to the Luxembourg-based European Court of Justice, the EU's highest court.
.@DouglasKMurray: "If judges are involved in politics, then politics will take an interest in the judges" #Murnaghan https://t.co/0DjBRrscJ3 — Murnaghan (@SkyMurnaghan) December 4, 2016
European Council President Donald Tusk has said that Britain might ultimately decide not to leave the European Union as the EU will not offer London softer terms than a damaging hard Brexit.
Tusk said his legal view was that if Britain unilaterally withdrew its request to leave before the two years were up, then it could stay in the Union.
Tusk said he had not found any national leader who wanted Britain to quit and so London would find a welcome if it changed its mind. "If we have a chance to reverse this negative process, we will find allies," he said. "I have no doubt."
The man who helped draft Article 50, John Kerr, a former British ambassador to the EU, said that Britain could still change is mind.
"You can change your mind while the process is going on," Kerr told BBC TV last month. "During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time."
#news #politics Lord Howard 'confident' of Brexit case success https://t.co/jG1igeCE3e pic.twitter.com/VUc2EN2R3v — WorldNewsPlayx (@WorldNewsPlayx) December 5, 2016
Both legal teams have hired recognised experts in international law. The government has Guglielmo Verdirame, a professor of international law at King's College, while the claimant have Dan Sarooshi, professor of public international law at the University of Oxford.
Other lawyers have argued that once the notification under Article 50 of the EU treaty is made, it can be rescinded and that the European Court of Justice could be consulted by the Supreme Court if there is uncertainty.
"It would be impossible if this issue of reversibility is argued before the Supreme Court for it not to be referred to the Court of Justice of the European Union," said Charles Streeten, a British barrister who has examined the legal arguments around the invocation and possible revocation of Article 50.
"It is a political judgement for the government: Win this case or deal with the public outfall of the question being referred to the Court of Justice of the European Union and see their Brexit timeline shattered," said Streeten, who is not involved in the Brexit case.
Fears and questions over Brexit’s impact on talent - While UK tech founders continue to try to second guess wh... https://t.co/vkCEsGJvhJ— Marco Filipe Novo (@mfcnovo) December 5, 2016