On Thursday morning, the UK High Court handed down a hell of a ruling: Prime Minister Theresa May’s government can’t withdraw from the European Union without first winning approval in the UK Parliament.

This is a huge, huge deal. June’s Brexit referendum was nonbinding, but most people assumed it had more-or-less decided the matter. The British people had voted to leave the European Union, and the government wouldn’t dare ignore that. Brexit was more or less inevitable; the only question was what the terms of the UK’s departure would be.

Not anymore. Now, May needs to win a majority vote in Parliament, and no one’s sure when it will happen or even what exactly members of Parliament would be voting on. Many MPs, including some from her own party, are deeply skeptical about leaving the EU because of fears it will weaken the British economy and weaken London’s standing on the world stage.

That being said, the British government is appealing the court’s decision, so it’s possible that the ruling gets overturned and a vote becomes unnecessary. So far, no major party has suggested that Parliament should outright block Brexit.

So I wouldn’t say that Brexit is dead, or even unlikely. But assuming the ruling stands, leaving the EU just got a lot more complicated — and, as a result, more likely to never happen in the first place.

The ruling makes Brexit much harder

The court ruling centered on something called Article 50 of the Lisbon Treaty, the provision governing how an EU member can leave the organization. Under that provision, an EU state has to provide formal notice to the union of its intent to quit. After formal notice, the country has two years to negotiate the terms of its exit — whether it will still have access to the EU’s free trade and free migration zone, for example — before it gets automatically booted out and all of those privileges get terminated.

Everyone agreed that the Brexit referendum did not qualify as an Article 50 notice. It was an internal, nonbinding UK vote — one that didn’t require the UK government to tell the EU anything. Afterward, the UK government held off on Article 50 notification, hoping that they could negotiate terms with the EU before starting the two-year countdown clock.

Initially, most people assumed that the prime minister’s office would be able to provide Article 50 notification on its own. But the High Court says, as a matter of constitutional law, that’s wrong — and only Parliament can do it.

The reason why is a bit technical, relating to the structure of the UK’s constitutional monarchy.

In the British system, the prime minister isn’t determined by direct election, but rather by a majority of members of Parliament. The crown — currently Queen Elizabeth — formally appoints the person who commands a parliamentary majority to be prime minister, and thus the head of government. This appointment confers executive powers, including the ability to conduct foreign policy, on the new prime minister.

But the powers the queen grants are limited. Crucially, they don’t include changing domestic UK law — only a vote of Parliament is supposed to be able to do that. Triggering Article 50 would necessarily end up changing UK law, which is currently written to accommodate EU rules. Hence, the High Court rules, only Parliament can do it.

“The most fundamental rule of the UK constitution is that Parliament is sovereign,” the court writes in a summary document explaining the ruling. “The government does not have the power under the Crown’s prerogative to give notice pursuant to Article 50.”

The tricky bit, though, is that it’s not clear what this actually means. Could Parliament delegate Article 50 powers to the prime minister? Does it have to pass a bill codifying every change to domestic law that would result from Brexit? Or is it something else entirely?

Nobody knows for sure, and it’s freaking Britain out.

“MPs are talking of the high court triggering a constitutional crisis without any indication of how to get out of it,” the Guardian’s Anne Perkins writes.

“No Brexit” is more possible than it used to be

This could all be a moot point. The UK Supreme Court is slated to hear the government’s appeal in December, and it’s possible it overrules the lower court’s decision.

But assuming the ruling stands, the process of Britain quitting the EU just got harder. Brexit is still likely to happen, but its odds just became a fair bit longer.

Parliament isn’t likely to block Brexit outright. The thinking right now is that thwarting the will of the British people, as expressed in the referendum, would be illegitimate and an act of political suicide for lawmakers voting to ignore the vote. Both May and her leading opponent, Labour leader Jeremy Corbyn, publicly opposed Brexit — and both have said that they respect the results of the referendum.

“There would in the Commons be — and I think, rightly — little or no enthusiasm for rejecting [Brexit],” Jolyon Maugham, a British lawyer who supported the court ruling, writes in the Financial Times.

What’s actually plausible, according to Maugham, is that Parliament exercises some kind of veto on the terms of Brexit. Basically, Parliament might only allow the government to provide Article 50 notification if it gets the EU to agree to preserving certain British privileges.

The most crucial one is access to the EU common market, its expansive free trade regime.

The EU is Britain’s biggest trading partner, by far. The UK economy depends heavily on financial services, which in turn depend on being able to move freely in EU financial markets. Losing privileged access to the EU would likely be economically ruinous for the UK as a result. The value of the pound has been in free fall for months, owing to widespread expectations of coming disaster.

The EU, for its part, has little reason to grant UK such privileges if it leaves. The EU’s interest is to make the Brexit process as painful as possible to ensure that other countries don’t follow Britain’s lead and leave the union. And they’re holding the cards: Britain needs access to the European economy more than Europe needs access to Britain’s.

This logic could give MPs a reason to block Brexit. They could claim to respect the referendum, in public, while crafting legislation that blocks a “hard Brexit” — one in which the UK loses access to the EU market. This might, in effect, end up stopping Brexit from happening.

Maugham conjures up a hypothetical to explain the logic:

Imagine this. It is November 2018. The promised trade deals have failed to appear. Unemployment and inflation are on the rise. The public mood towards Brexit has turned ugly, and so have negotiations with our biggest and nearest trading partner. In that world, any rational MP would wish that, back in November 2016, she had left ajar the door to remaining in the EU. Why make the decision now, with modest evidence, if you have the opportunity to make it later with more?

Another possibility is that Parliament refuses to ratify the terms of Brexit without a vote — either a second referendum or a parliamentary election. That wouldn’t be outright blocking Brexit, but it would be punting the issue to British voters, who might very well change their minds as the consequences of the vote become clear.

Is any of this likely to prevent Brexit outright? Most British political observers right now are saying no: The UK’s leading politicians, after all, are all publicly saying that Brexit is still happening.

“Can this stop Brexit? Almost certainly not,” Perkins writes. “But it does make the position much more confused.”

And in the midst of this confusion, it’s possible that things go awry for the pro-Brexit camp. This situation is unprecedented, and nobody expected the referendum to win in the first place. It’s possible, even if not likely, that things could take yet another unpredictable turn.