In a landmark decision on Tuesday, the Supreme Court in the UK ruled that the British government cannot trigger Article 50 to officially notify the European Union of Brexit without the approval of Parliament.
The judges decided by a majority of 8-3 that the UK's constitutional arrangements require such a fundamental change that they would need to be clearly authorised by Parliament.
The ruling upheld the campaigners' claim that it would be undemocratic for the government alone to decide to trigger Article 50, and that Parliament and the House of Lords must have a vote because the move would mean overturning existing UK laws.
The government had appealed against a similar decision by the High Court, insisting that under the Royal Prerogative it did not need to consult Parliament and that MPs had already given their authorisation when they supported the EU referendum being held last year.
However, the Supreme Court judges also decided unanimously that the government does not have to consult the Scottish Parliament and the Welsh and Northern Ireland assemblies about triggering Article 50.
On Wednesday, prime minister Theresa May announced that the government will, after all, be publishing a White Paper to summarise its plans for the Brexit negotiations. She did so under pressure from MPs of all parties to provide more information about what the government wants to happen once Article 50 has been triggered. Some MPs had insisted that they would be unable to vote on triggering Article 50 if they did not know the details of the government's strategy. However, it is not known when the White Paper, which is likely to be a lengthy document, will be published and it is possible that Mrs May will decide to hold the vote on triggering Article 50 beforehand.