"Theresa May travels to Brussels on Wednesday to ask European leaders for a delay to Brexit. The prime minister is widely expected to agree an extension to Article 50, the formal exit process. However, with differences over how long the extension should be, and what conditions would be attached, there is a possibility that European leaders and Mrs May are unable to reach an agreement. That would leave the UK with two options: a no-deal Brexit at 11pm London time on Friday; or revoking Article 50, thereby stopping the Brexit process. Given that parliament has now legislated to prevent a no-deal Brexit, Mrs May would arguably only have one choice: revocation. What is revocation? Stopping Brexit, at least in the short term. The UK began the formal process of leaving the EU in March 2017 by notifying the bloc of its intention to leave, as envisaged by Article 50 of the EU Treaty. Revocation means taking back this notification so that the UK remains a EU member state on current terms — without changes to its budget contributions or veto powers. Does the EU need to agree to revocation? No. The European Court of Justice ruled in December that a member state can unilaterally revoke a decision to leave — although the UK government and the European Commission had both argued the contrary. By contrast, extending Article 50 requires the unanimous agreement of all EU member states. Legally, how would Mrs May revoke Article 50? The ECJ laid down four relevant conditions. First, the revocation must be “submitted in writing to the European Council” of leaders of EU member states. Second, the revocation must occur before the end of the Article 50 process. So, if the UK left the EU on Friday without a deal, it could not revoke on Saturday. Third, the revocation must be “unequivocal and unconditional”. The meaning of this is debated, but Jolyon Maugham, a barrister who helped to take the issue of revocation to the ECJ, argues that revocation would be invalid if the UK simply intended to restart the Brexit process shortly afterwards. The ECJ’s fourth requirement is that revocation must be made “in accordance with [the member state’s] constitutional requirements.” This is again contested. In January 2017, the UK’s supreme court ruled that the prime minister could not invoke Article 50 without an act of parliament, following a legal challenge by the campaigner Gina Miller. There is an argument that Mrs May would require another act in order to revoke Article 50, which would be a challenge in less than 48 hours. Mr Maugham has argued that the prime minister retains the prerogative power to revoke without consulting parliament. But he suggests that the prime minister could organise a vote among MPs in any case. If MPs have voted for revocation, it would be harder for a Eurosceptic politician to bring a legal challenge saying that the decision was invalid. “Is a court really going to say, contrary to the wishes of parliament, we are not any longer in the EU?” said Mr Maugham. Politically, would revocation be plausible? More than 6m people have signed a petition calling for revocation. In one opinion poll, revocation is the single most popular Brexit option, with 40 per cent support. But even Europhile MPs have been loath to end the Brexit process without another referendum. A motion by the Scottish National party MP Joanna Cherry, which would have set a path to revocation if the alternative were a no-deal Brexit, was defeated by 292 votes to 191 last week. Significantly only nine Conservative MPs voted in favour of it. Mrs May would therefore have a battle to win the support of parliament, and would face an even greater revolt among her own party. Perhaps, however, the threat of a no-deal Brexit within hours would focus MPs’ minds. What would happen after revocation? This depends on Mrs May and parliament. It would almost certainly not bring Brexit to an end. But it would give the UK the chance to have a general election, another referendum or even citizens’ assemblies — to decide on a way forward without a fixed legal deadline. That could be the end of Brexit, or it could be the prelude to another Article 50 notification and another two years (or more) of negotiations."