The relationships between the United Kingdom and Europe are characterized by ambiguities and uncertainties. The Brexit process suffers the same fate but raises important questions of constitutional law. The Miller case highlights how the sovereignty of Parliament plays a major role in the British constitutional landscape. A statute is necessary if the United Kingdom wants to leave the European Union has held the Supreme Court. Examining the content of the withdrawal, many parliamentary committees have insisted on the importance of EU citizens’ rights. Indeed, the Rule of Law can be affected by a decision of the United Kingdom to repeal rights of citizens’ deriving from European Union law. While changing British constitutional law, the Brexit process could also give a new momentum to rethink the constitutional future of the European Union.