Byline: Gavin Phillipson This article analyses the Article 50 TEU debate and the argument that for the UK Government to trigger the formal withdrawal process without explicit parliamentary authorisation would be unlawful, because it would inevitably result in the removal of rights enjoyed under EU law and the frustration of the purpose of the statutes giving those rights domestic effect. After a brief survey of Article 50, this article argues first of all that the power to trigger Article 50 remains within the prerogative, contesting Robert Craig's argument in this issue that it is now a statutory power. It then suggests a number of arguments as to why the frustration principle may be of only doubtful application in this case, and in doing so it re-examines one of the key authorities prayed in aid of it - the Fire Brigades Union case. Article Note: Durham Law School. The author would like to thank Robert Craig, Colm O'Cinneide, Paul Craig, Carl Gardener, Alison Young, Robert Schuetze and the anonymous reviewer for comments on an earlier draft and Jo Murkens for valuable discussions on relevant EU law points; any remaining errors are the responsibility of the author. All websites cited were last accessed on 12 September 2016.