The High Court has ruled that the Prime Minister must seek the approval of MPs in order to trigger the process of taking Britain out of the EU - something Remainers are very happy about.
This is what it all means for the future of the process of Brexit:
What happened in the court case?
In one of the most important constitutional cases in generations, three senior judges ruled that the Prime Minister does not have power to use the royal prerogative to trigger Article 50 of the Lisbon Treaty to start the two-year process of negotiating Brexit without the prior authority of Parliament.
What is the royal prerogative?
This is a set of powers given to the PM and her Government which would enable her to make decisions without consulting and getting the backing of parliament.
Government lawyers had argued that prerogative powers were a legitimate way to give effect "to the will of the people" who voted by a clear majority to leave the European Union in the June referendum.
But Lord Thomas declared: "The Government does not have power under the Crown's prerogative to give notice pursuant to Article 50 for the UK to withdraw from the European Union."
Why did this even go to court?
When Theresa May announced at the Conservative Party conference that she intends giving an Article 50 notification by the end of March 2017, "concerned citizens" from all walks of life came forward to challenge her, including investment fund manager and philanthropist Gina Miller.
What will the Government do next?
International Development Secretary Liam Fox told the House of Commons within minutes of the ruling that the Government will appeal to the Supreme Court.
A Government spokesman said: "The Government is disappointed by the Court's judgment. The country voted to leave the European Union in a referendum approved by Act of Parliament. And the Government is determined to respect the result of the referendum. We will appeal this judgment."
What does that mean for Brexit if the appeal is overturned?
The ruling threatens to plunge the Government's plans for Brexit into disarray as the process will have to be subject to full parliamentary control.
How have politicians reacted to the news?
Opponents of a "hard Brexit" welcomed the news. Liberal Democrat leader Tim Farron said the Government must now lay out its negotiating position in Parliament, while Scottish First Minister Nicola Sturgeon described the finding as "significant indeed".
Green co-leader Caroline Lucas hailed it as "brilliant news", adding: "That's what taking back control should be about - better democracy."
But outgoing Ukip leader and leading Brexiter Nigel Farage tweeted: "I worry that a betrayal may be near at hand" shortly after the ruling.
And the public?
Campaigners have been celebrating the legal victory, while many people on Twitter taking joy from the fact if the appeal is overturned, they could then always appeal further to the European Court of Justice in Luxembourg.