By Rebecca Pinnington

Until the Lisbon Treaty in 2009, there was no official mechanism for withdrawal from the European Union. Even as Article 50 came into effect, the idea of leaving the EU was highly controversial. It was never really meant to be used. Now, an article less than 300 words in length forms the basis of an increasingly fraught constitutional and political debate.

How Article 50 can be triggered will be decided by the Supreme Court in the new year. Whether it can be un-triggered if Britain changes its mind is less clear. Legally, there are arguments to suggest this is possible. Politically, however, the situation is more complex.

Here are the views of four European law experts on what could soon become a matter of vital importance.

Stefan Enchelmaier, professor of European and Comparative Law at Oxford University, argues that the essential requirement of withdrawing from the EU is the simple desire to leave. "If you look at Article 50, first you have the decision to withdraw, then you have the intention to withdraw," he says. "No withdrawal without notification, but, the other way round, no withdrawal without the intention to withdraw." If public opinion reversed and the UK wanted to remain, therefore, this would legally be possible.

Kenneth Armstrong, professor of European Law at Cambridge University, says this is the "dominant legal opinion" on the subject and the authors of the Article are in agreement.

However, Article 50 is not very well written. There is no explicit provision for this. Equally there is no precedent for withdrawing from the European Union, so there is little to go on. Jan Komarek, a lecturer at the London School of Economics' European Institute and Department of Law, says that, as Article 50 is "silent" on whether a withdrawing member state could change its mind during the negotiation period, lawyers would have to look for other examples in international law. Under the Vienna Convention, for example, a "fundamental change of circumstances" is grounds for withdrawing from an international agreement, provided "the existence of those circumstances constituted an essential basis of the consent of the parties to be bound by the treaty".

In other words, if the UK no longer consents to being part of the EU, it may leave the EU. If that situation changes and the UK decides to remain, it may remain. "When a state which is party to an international agreement wants to revoke the agreement and then wants to revoke the revocation of the agreement, that is possible according to the Vienna Convention," Komarek says.

Enchelmaier also refers to contract law in explaining why triggering Article 50 should be reversible. Let's say you want to terminate a contract. "Your declaration cannot be undone, because once you've declared the contract is done, it's void, there's nothing left. The declaration of your intention that you want to withdraw, the notification under Article 50 paragraph two, is obviously not of that nature because it is followed by negotiations."

There's an important political consideration, however, in that the EU will not want the UK to change its mind again and again until negotiations go its way. "They might want to get some sort of commitment or agreement from the UK that that is a definitive view," Armstrong explains. "They may want to then see that there is actually a commitment to withdrawing the notification, because they don't want to see the timetable effectively subverted by the UK trying to pause on negotiations by withdrawing the notification."

"The EU's position is likely to be that the UK can change its mind. I don't think anybody in the EU wants the UK to leave, and so I don't think they would necessarily put up any obstacles to the UK revoking its notification."

The ongoing Article 50 Supreme Court case further complicates matters. The High Court judgment on the case explicitly states that "a notice under Article 50 cannot be withdrawn, and Article 50 does not allow for a conditional notice to be given". The claimants have based their case on the assumption that it is irrevocable, and at least in the High Court, the government agreed.

Camino Mortera-Martinez, a research fellow specialising in justice and home affairs at the Centre for European Reform, emphasises that the debate is not purely legal, it is "a combination of politics and the law". So while the article in itself is probably revocable, "the Supreme Court interpretation is likely to be that Article 50 is non-revocable, and therefore any case for revocability is going to be a lot harder to make". But this isn't a British legal matter. "The question of whether Article 50 is revocable is European Union law and cannot be decided by the national courts, it's a question for the European Court of Justice," she adds. "But it is unlikely to be asked to pass judgement because the two parties are agreed."

It's unlikely there will be a definitive answer on this question any time soon - not before Article 50 is triggered, and probably not unless the government decides to pursue a revocation of its revocation of EU membership. And that in itself does not seem a very realistic process. But a lot can change in two years. If it does, the experts generally agree that the decision to leave the European Union can be undone.

Rebecca Pinnington is a freelance journalist. Follow her on Twitter.

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