On 4 December 2018, the ECJ’s Advocate General gave his opinion that Article 50 is revocable and consequently that the UK could change its mind before any Exit Day. The ECJ agreed with the Advocate General in its ruling, interpreting Article 50 in accordance with the wider EU treaties and their purpose, the origin of Article 50, settled case law, and by analogy to the principles of public international law. It held that revocation, like notification, is an act of state sovereignty and must be possible because no member state can be ejected from the EU against its will.
The ECJ noted that the conditions on any revocation would be the same as those stated in Article 50 for the original notification. First, the decision must be taken in accordance with the member state’s constitutional requirements. Secondly, the decision must be notified to the Council in writing thereafter.
ukandeu.ac.uk/revoking-article-50-after-the-ecjs-ruling/