Judges have ruled that parliament must be given a vote before the government can start the formal process of exiting the EU. The legal decision could intensify the divide between "remainers" and leavers.
"Today we found out that those who want to 'restore parliamentary sovereignty' have not succeeded in denying parliament a vote," lawyer and legal commentator David Allen Green said. "At first glance, this unanimous, detailed and reasoned judgment, by three very senior judges, looks appeal-proof."
However, the government is expected to appeal the decision at the Supreme Court, which has reserved time for a hearing on December 7 and 8.
Prime Minister Theresa May had promised to trigger Article 50 of the Lisbon Treaty by the end of March 2017 and her government insisted that it did not need to consult parliament before beginning this two-year process. Numerous MPs argued that the government did not have a mandate to do this, but the administrators maintained that these MPs were trying to "subvert democracy" by delaying the process of Brexit.
Campaigners, led by investment manager Gina Miller, brought a case to the High Court arguing that the government did not have the power to invoke Article 50 without approval from MPs. The judges ruled in their favor.
"May has said that this is a matter for government, but in the end that opinion is irrelevant," Oliver Patel, research associate at University College London's European Institute said. "One thing this country is good at is having a very solid judicial system and rule of law, so ultimately, the government has to do what the court says."
Yet if anything, the decision is likely to fuel the division between "remainers" and leavers. Suzanne Evans, who is standing for leadership of the UK Independence Party (UKIP) called for the judges to be sacked, accusing them of attempting to "overturn our will" and "undermine democracy." UKIP's current leader Nigel Farage warned that "a betrayal may be near at hand."
The three-day hearing saw arguments made, which, at the very least, ramped up political pressure for parliament to be given a greater role in Brexit negotiations.
"This decision has huge implications, not just on the timing of Brexit, but on the terms of Brexit," BBC political correspondent Eleanor Garnier said. "That's because it's given the initiative to those on the Remain side in the House of Commons who, it's now likely, will argue Article 50 can only be triggered when parliament is ready. And that could mean when they're happy with the terms of any future deal."
The judgment does not make it clear whether this would be a short, sharp vote, or whether parliament would need to consider complex legislation. However, if the ruling is upheld at the Supreme Court, it could mean months of delays as various parliamentary hurdles must be passed.
The Conservatives have a majority in the House of Commons of just 12 seats, and many of those MPs campaigned and voted for Remain. While most MPs will not want to appear to be blocking the referendum result, it could be difficult for the government to get agreement from a majority of MPs to trigger Article 50.
"Parliament would request information on the government's negotiating position before they passing a vote, so the government would have to reveal what it's plans are," researcher Oliver Patel said. "It definitely decreases the likelihood of a super-hard Brexit, whatever that is, because it's hard to see parliament voting for the UK to come fully out of the single market. Hardly anyone in parliament actually wants that bar a few right-wing Tories," he added.
It has been speculated that this decision - if upheld - could make an early general election more likely.
After the Supreme Court, the next recourse would be to the European Court of Justice (ECJ), but given the current political climate in Britain, there is little appetite on any side for a European institution to be seen deciding on an aspect of the British constitution.