In a ruling released yesterday morning, three senior judges of the High Court of Justice of England and Wales (the “High Court”) handed the UK Government a significant defeat in relation to its Brexit plans. The High Court ruled that the UK Government could not use executive powers under the ‘Royal prerogative’ to trigger the mechanism for the UK to exit the European Union (the “EU”). Instead, assuming the UK Government’s appeal to the UK Supreme Court is unsuccessful, the UK may only begin the process of Brexit following UK parliamentary approval. Although this development is unlikely to lead to a reversal of Brexit, it could well have significant implications for both the timetable and the terms. The decision has the potential to cause delay and will subject the UK government’s negotiating position to more parliamentary scrutiny and debate.