The Supreme Court has ruled against the government, insisting that MPs must vote before Theresa May can trigger the UK's exit from the European Union.
Delivering the landmark judgement on Tuesday morning, the Court's president Lord Neuberger, said the decision was agreed by eight to three on the panel of 11.
"Supreme Court holds that an Act of Parliament is required to authorise ministers to give notice of the decision of the UK to withdraw from the European Union," he said.
The focus of the judgement, read out by Lord Neuberger, said that because the UK would change as a result of leaving the EU, Parliament must be consulted.
"Withdrawal makes a fundamental change to the UK's constitutional arrangements, by cutting off the source of EU law," the ruling stated. "Such a fundamental change will be the inevitable effect of a Notice being served.
"The UK constitution requires such changes to be effected by Parliamentary legislation," it concluded.
The government's lawyers had argued ministers had the power – through their royal prerogative – to enact Article 50 without a vote from MPs. They said that parliament was already consulted when it passed the bill to hold a referendum and had not asked for a final say.
But opponents insisted parliament must be consulted before such a wholesale change in the UK's constitution and the two years of negotiations over the terms of exit begin.
Ministers had anticipated their loss and Theresa May moved to nullify the effects of the legal defeat by granting that Parliament will be consulted on the final shape of any Brexit deal.
Brexit secretary David Davis is expected to outline the government's next steps in a statement to the Commons and the government is thought to draft a very simple piece of legislation to trigger Article 50 before March.
Although the government lost the legal case, it is thought the Commons will easily pass a bill to trigger Article 50 with all 329 Conservative MPs backing the move. Labour leader Jeremy Corbyn has told MPs to approve the bill and it is thought, despite dozens rebelling, that most will fall in line.
SNP and Lib Dem MPs have vowed not to back the bill.
The panel of 11 justices is the largest ever for a single case. Normally the Court sits in panels of five.
The Supreme Court judgement insisted it did not affect whether the UK should leave the EU and was not a political judgement.
"The issues in these proceedings have nothing to do with political issues such as the merits of the decision to withdraw, the timetable and terms of so doing, or the details of any future relationship between the UK and the EU," it read.
It comes after High Court judges were accused of blocking Brexit and were branded "enemies of the people" after the initial ruling that MPs must vote on triggering Article 50.
The judgement added that all 11 judges agreed unanimously that the government did not have to consult the devolved administrations of Scotland, Wales and Northern Ireland before triggering Article 50.
