THE High Court has ruled that Parliament must hold a vote on Britain's membership of the European Union before it can trigger Article 50.
This means that the Government cannot trigger the exit and begin formal discussions to leave the EU on their own.
Campaigners have declared that leaving the EU purely on a referendum vote is unconstitutional calling today's verdict "a victory for parliamentary democracy".
Lord Chief Justice Lord John Thomas said: "The court does not accept the argument put forward by the Government. There is nothing in the text of the 1972 Act to support it.
"In the judgment of the court the argument is contrary both to the language used by Parliament in the 1972 Act and to the fundamental constitutional principles of the sovereignty of Parliament and the absence of any entitlement on the part of the Crown to change domestic law by the exercise of its prerogative powers. The court expressly accepts the principal argument of the claimants.
"For the reasons set out in the judgment, we decide that 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."
It comes less than a week after a Northern Irish judge rejected challenges to the Brexit process, pushing the issue to the UK's top court to decide on whether Parliament needs to be involved.
Prime minister Theresa May had previously said that Article 50 triggered by the end of next March but this ruling could now see a further protraction of events and yet more uncertainty.
The UK voted to leave the EU in June's referendum by a margin of 51.9% to 48.1%.
The EU has been steadfast in saying that the two-year negotiations over the terms of the UK's exit cannot begin until Article 50 has been invoked.
The Government is expected to appeal the decision.