Article 50 states that "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements." It's the "constitutional

Article 50 states that "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements." It's the "constitutional requirements" clause that's the problem. Britain does not have a written constitution, but it has traditions that are considered an unwritten constitution, eg. the government (ie. the ruling party) cannot take away rights of the citizens without the consent of Parliament. How exactly this interacts with the Brexit vote is a difficult question, hence the need for this court case.