The High Court in London has stated that the British government requires approval from parliament to trigger 'Brexit' - the process of exiting the European Union.
The government said it would appeal against the decision and a spokeswoman for Theresa May said the prime minister would press ahead with planned talks on the Brexit terms by the end of March.
Lord Chief Justice John Thomas stated, "The most fundamental rule of the UK's constitution is that parliament is sovereign and can make and unmake any law it chooses."
Parliament now is able to block Britain’s exit from the European Community in the face of 52% of the British people having voted in the June referendum to leave the EU.
The ruling puts at risk Theresa May's deadline for triggering Article 50 of the EU's Lisbon Treaty, the formal step that needs to be taken to start Britain’s withdrawal.
A spokesman for the British Prime Minister said this morning, "Our plan remains to invoke Article 50 by the end of March, we believe the legal timetable should allow for that."
Nigel Farage stated on Twitter that he feared the ruling might evolve into an attempt to avoid Brexit altogether.
"I worry that a betrayal may be near at hand. I now fear every attempt will be made to block or delay triggering Article 50. They have no idea of the level of public anger they will provoke."
Farage feared a betrayal of the 51.9% of voters who backed leaving the EU in June's referendum and voiced concern at the prospect of a "half Brexit".
The other 27 member states have said negotiations about the terms of the UK's exit, due to take two years, cannot begin until Article 50 has been invoked.
The Lord Chief Justice 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."
The three judges looking at the case found that triggering Article 50 would fundamentally change UK people's rights and that the government cannot change or do away with rights under UK law unless Parliament gives it authority to do so.
Calling the case "a pure question of law", Lord Thomas said, "The court is not concerned with and does not express any view about the merits of leaving the European Union - that is a political issue."
But Liberal Democrat leader Tim Farron said: "Ultimately, the British people voted for a departure but not for a destination, which is why what really matters is allowing them to vote again on the final deal, giving them the chance to say no to an irresponsible hard Brexit that risks our economy and our jobs."
