In answer to the question from the Scottish court, the Advocate General proposes that the Court of Justice should, in its future judgment, declare that Article 50 TEU allows the unilateral revocation of the notification of the intention to withdraw from the EU, until such time as the withdrawal agreement is formally concluded.
As the issue of revocation is not expressly provided in Article 50 TEU, to interpret the article, the Advocate General refers to the relevant provisions of the Vienna Convention on the Law of Treaties on which Article 50 TEU is based. Pursuant to Article 68 of that convention, notifications of withdrawal from an international treaty may be revoked at any time before they take effect.
However, that possibility of unilateral revocation is subject to certain conditions and limits. First, like the notification of the intention to withdraw, the unilateral revocation must be notified by a formal act to the European Council. Secondly, it must respect national constitutional requirements. If, as is the case in the UK, prior parliamentary authorisation is required for the notification of the intention to withdraw, it is logical that the revocation of that notification also requires parliamentary approval. There is also a temporal limit on the possibility of revocation, since revocation is possible only within the two-year period that begins when the intention to withdraw is notified. The principles of good faith and sincere cooperation must also be observed, in order to prevent abuse of the procedure laid down in Article 50 TEU.
However, the Advocate General considers that to make the possibility of revocation conditional upon the adoption of a unanimous decision of the European Council would be incompatible with Article 50 TEU.
For more detailed information, please see the press release of the Court of Justice.