The High Court has ruled that Triggering Article 50 will require the consent of MPs and parliament today.
Senior judges ruled that Prime Minister Theresa May (pictured) does not have the power to bypass MPs and start the clock on a two-year window to leave the EU.
The government will appeal against the judgement to the Supreme Court and the hearing will take place early next month.
May had pledged to trigger Article 50 by the end of March 2017.
Government lawyers argued that prerogative powers were a legitimate way to carry out the “will of the people”.
But the Lord Chief Justice is quoted in The Telegraph as saying the government’s arguments are “contrary to fundamental constitutional principles of the sovereignty of parliament”, before adding “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”.