This paper presents an analysis of the implications and the consequences of the UK’s decision to withdraw from the EU in the context of the principle of transparency and the UK Government’s accountability in the Brexit negotiations. In political terms, the result of the 2016 Brexit referendum took on a particular role because its effects made it impossible for the parliament to disregard the will of the people and in legal terms, the Government was unable to trigger Art. 50 of the TUE. The outcome of the so-called Miller I case showed that the courts had been forced again to draw the boundaries of constitutional competence between the executive and the parliament, in the sense that they had consistently backed Westminster. Thus, this paper discusses that the credibility of the EU and the UK Government depended on the principle of transparency in the Brexit negotiations. The author argues that openness is a key element in ensuring accountability in the decision-making process.