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Theresa May’s decision to formally trigger Britain’s Article 50 request to leave the EU in March 2017 has a huge unanswered question at its heart: Whether the request is
reversible before the two-year deadline elapses.

That sounds like a technicality. But it could change everything. If the UK can withdraw its request halfway through the Article 50 process, then this gives Britain some bargaining power in the Brexit negotiations: If we don’t get what we want, we can walk away or force the process to start over again.

But if Article 50 cannot be reversed then all the power lies with Brussels: The EU need not bother to negotiate at all — it could simply sit on its hands while the two-year clock runs down, ejecting Britain in a “hard Brexit.” An Article 50 trigger might be a trap, in other words, with only one way out.

The difference between the two positions is massive: If it is reversible, then the March trigger will start a genuine round of talks, the outcome of which is uncertain. The UK can argue to get the best deal possible.

If it is not reversible, then the March trigger is the same thing as leaving the EU.

Until recently, it had been assumed by many — including Business Insider — that an Article 50 request could be formally withdrawn any time before the two-year period ended. The House of Lords took legal advice on the issue in April 2016 and was told:

“It is absolutely clear that you cannot be forced to go through with it if you do not want to: for example, if there is a change of Government … Analysis of the text suggests that you are entitled to change your mind. … There is nothing in Article 50 formally to prevent a Member State from reversing its decision…”

That advice dovetails with the opinion of former director-general of the Council of the European Union’s Legal Service Jean-Claude Piris, who wrote in September that “Even after triggering Article 50 and notifying the EU of its intention to leave, there is no legal obstacle to the UK changing its mind, in accordance with its constitutional requirements.”

But the legal arguments in the “People’s Challenge” case heard today at the High Court raised the issue. It turns out there is no rule or precedent explicitly stating whether Article 50 is reversible or not. Barrister Jolyon Maugham has an informative blog post on the issue here. And the Financial Times’ David Allen Green outlines the issue similarly here.

It is not clear whether the judges’ ruling in the case will state a position on reversibility.

Maugham notes that the reversibility issue carries huge political consequences. The High Court case is mostly about whether Parliament should vote to formally trigger Article 50, or can the government do it on its own without a vote?

If the trigger is reversible, that would imply Parliament — or a new referendum — could vote to reverse the trigger. In other words, even if Prime Minister Theresa May did not seek approval from the House of Commons to trigger Article 50, the House could still cast a vote later, before the two-year period was up, yanking Britain out of the negotiations.

That holds up the tantalising possibility for Remainers that parliament may get more than one opportunity to block or delay Brexit.

In the more immediate future, Theresa May’s government needs to figure out whether Article 50 can be reversed before March arrives. Because if it is not reversible, then she will go into those talks with very few weapons indeed.