"Article 50 TEU must be interpreted as meaning that, where a Member State has notified the European Council, in accordance with that article, of its intention to withdraw from the European Union, that article allows that Member State — for as long as a withdrawal agreement concluded between that Member State and the European Union has not entered into force or, if no such agreement has been concluded, for as long as the two-year period laid down in Article 50(3) TEU, possibly extended in accordance with that paragraph, has not expired — to revoke that notification unilaterally, in an unequivocal and unconditional manner, by a notice addressed to the European Council in writing, after the Member State concerned has taken the revocation decision in accordance with its constitutional requirements. The purpose of that revocation is to confirm the EU membership of the Member State concerned under terms that are unchanged as regards its status as a Member State, and that revocation brings the withdrawal procedure to an end."
Such is the conclusion of the European Court of Justice in a preliminary Ruling published on 10 December 2018. In its argument, the Court rejected the submission on behalf of the British Government that the question was purely hypothetical, as well as the submission on behalf of the EU Council that a unilateral revocation would be contrary to European Law.
A truly supreme irony, that the the current British Government might be saved by the Court, the jurisdiction of which had been one of the principal reasons for Brexit.