Can Britain's EU divorce be reversed once it is triggered?

LONDON, Dec (Shanghai: 600875.SS - news) 5 (Reuters) - 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.

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WHAT DOES BRITAIN SAY?

Attorney General Jeremy Wright, the British government's top lawyer, told the High Court on Oct (HKSE: 3366-OL.HK - news) . 17: "We do not argue that an Article 50 notice can be revoked.

"We invite the court to proceed in this case on the basis that a notification under Article 50(2) is irrevocable," he said.

Prime Minister Theresa May's spokeswoman said the case is about constitutional law in the United Kingdom and not about a dispute over EU law so it will be decided in the Supreme Court.

"The case is about a question around constitutional law in the UK, it's not about a disputed point of EU law. That's not what's at issue, so the expectation is it will be decided by the Supreme Court," May's spokeswoman said on Nov. 3.

On Nov. 30, May's spokeswoman said it would not go to the European Court of Justice: "It's not within scope, it's not an issue for the European Court to rule on."

WHAT DOES THE EU SAY?

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."

WHAT DO THE LAWYERS SAY?

In England's High Court, lawyers for a group of claimants who challenged the government to allow parliament to decide when and how to trigger Article 50 said that it was irrevocable. The government's lawyers agreed. Lawyers for both sides asked the court to assume that it was irrevocable.

In its final judgement, the High Court's judges said that once notice of leaving was given then it will "inevitably result in the complete withdrawal of the United Kingdom".

Ahead of the Supreme Court hearing, lawyers for both sides have argued that Article 50 is irreversible and both sides said it was irrelevant to the outcome of the case.

Government lawyers argue in their summary that once Article 50 is given, the UK will eventually withdraw from the EU. They note that it was common ground between the parties that Article 50 is irrevocable and cannot be given conditionally.

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 (Taiwan OTC: 4949.TWO - news) 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. (Reporting by Guy Faulconbridge Editing by Jeremy Gaunt)