Theresa May wanted to trigger the process without a vote in the House of Commons
British Prime Minister Theresa May cannot trigger Brexit without putting it to an MPs' vote in the House of Commons, the UK High Court has ruled.
In a landmark ruling, Lord Chief Justice Thomas said Mrs May did not have the right to set in motion Article 50, the official start of the two-year EU divorce proceedings, without consulting parliament.
The government instantly announced it would appeal the decision and the two sides will now prepare for another showdown at the Supreme Court in early December.
The British Supreme Court has earmarked December 7th and 8th to hear the appeal case.
The decision is a significant setback for Mrs May's Brexit strategy - she announced at the Conservative Party Conference last month she would trigger Article 50 by the end of March.
Speaking outside the court, businesswoman Gina Miller, who brought the case with hairdresser Deir Dos Santos, welcomed the decision and said it would "bring sobriety" to Brexit proceedings.
In a statement from Mr Dos Santos, who voted to leave the EU, he said: "In her speech to the Conservative Party Conference the Prime Minister attacked me for bringing these proceedings as a claimant. She said that I was trying to subvert democracy. That was an unwarranted and irresponsible attack.
"As is my constitutional right I sought the protection of the court to stop unlawful government action. The court has now given me that protection."
The ruling saw the pound sterling up more than 1% against the US dollar, at $1.24, in the immediate wake of the announcement.
Unless the decision is overturned then it will be for MPs to decide when to start the UK's exit from the European Union.
While many may be reluctant to overturn the public's decision, there will be a number in constituencies where people voted Remain who will come under pressure from their voters.
British Trade Secretary Liam Fox told Westminster the government was 'disappointed'.
"The government is disappointed by the court's judgement.
"The country voted to leave the European Union in a referendum approved by act of parliament - the government is determined to respect the result of the referendum".
The case has centred around the wording of Article 50 of the Lisbon Treaty, which says member states may leave the EU "in accordance with its own constitutional requirements".
However, there is no clearly established "constitutional requirements" leaving both sides free to make their own definitions.
Making his judgment, the Lord Chief Justice said: "The Government does not have power under the Crown's prerogative to give notice pursuant to Article 50 for the UK for the UK to withdraw from the European Union."
Scottish First Minister Nicola Sturgeon, who has said she may trigger a second independence referendum before the Brexit process begins, called the ruling "significant":
Significant indeed! https://t.co/ELtJ2tqknF— Nicola Sturgeon (@NicolaSturgeon) November 3, 2016