Article 50 of the Treaty on European Union (TEU) may be unilaterally withdrawn by the member State which gave the notice. Please read the court's announcement (pdf) which is available via the CJEU website.
The FULL opinion containing legal reasoning is also available.
The Advocate General’s Opinion is not binding on the Court of Justice. It is the role of the Advocates General to propose to the Court, in complete independence, a legal solution to the cases for which they are responsible. The Judges of the Court are now beginning their deliberations in this case. Judgment will be given at a later date.
The opinion is contrary to the view put forward by the European Commission in its "State of Play" document of 12 July 2017. In this previous post of 23 July 2017 the Commission's view was challenged. Please see the links to material by the various commentators both for and against unilateral revocation.
Note the conditions and limits proposed by the AG. 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.