The High Court has ruled that Prime Minister Theresa May cannot trigger Article 50 without putting it to an MPs' vote in the House of Commons.
Lord Chief Justice Thomas ruled that May did not have the right to set in motion Article 50, the official start of EU divorce proceedings, without consulting Parliament.
May had previously stated that the UK government will activate Article 50, formally notifying the EU of the UK's intention to leave, by the end of March 2017.
Thomas 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."
A government spokesperson 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."