In a non-binding opinion delivered to the Court of Justice of the EU (CJEU), advocate general Manuel Campos Sánchez-Bordona said the UK can reverse its formal decision to exit the EU, which it took when triggering Article 50 of the Lisbon Treaty, on its own, under certain conditions.
According to Campos Sánchez-Bordona, the UK's powers to unilaterally revoke Brexit only apply if the revocation decision is made in line with the UK's constitution, is formally notified to the European Council and that the principles of "good faith and sincere cooperation" are observed.
Kirsti McKenzie of Pinsent Masons, the law firm behind Out-Law.com, said: "This opinion, though not definitive on the case, will concern lawyers at the European Commission who fear that EU Member States could use the revocation of membership in future negotiations to gain more favourable terms. The 'good faith' and 'sincere cooperation' qualifications that Campos Sánchez-Bordona has suggested apply will provide them with a degree of comfort that other countries will not seek to play a game of bluff."
"For businesses, the opinion does not provide any greater clarity on Brexit. Indeed, if anything it may further confuse things for those politicians opposed to the UK government's Brexit deal to push for what at one stage may have only seemed a hypothetical outcome. With this being an expedited case, we might expect the CJEU's formal ruling prior to next week's Commons vote on the withdrawal agreement that the government has negotiated with the EU27," she said.
The European Commission and European Council have argued that Article 50 requires the unanimous agreement of the European Council for revocation of Article 50 to apply. They have raised concerns in particular that unilateral powers to revoke Article 50 could be used as a bargaining chip in Brexit negotiations. Campos Sánchez-Bordona acknowledged the weight of those arguments, but said the concerns were not "decisive".
"It appears to me that, in practice, it would be extremely difficult for tactical revocations to proliferate, undermining a possibility which undoubtedly has serious consequences," Campos Sánchez-Bordona said. "The revocation is a decision that the departing member state has had to adopt in accordance with its constitutional requirements."
"Since it entails the reversal of a previous decision of a constitutional nature, the change would require an alteration of the governing majority, the holding of a referendum, a ruling by the highest court of the country annulling the withdrawal decision or some other action that would be difficult to implement and would require the use of protracted and complex legal procedures," he said.
"The obligation that a revocation must be carried out in accordance with the member state’s constitutional requirements is thus a filter which acts as a deterrent in order to prevent the abuse of the withdrawal procedure … through such tactical revocations," he said.
According Campos Sánchez-Bordona, the way in which the right of withdrawal in Article 50 is expressed is "based on the rules of international law", and that the sovereign right of states to unilaterally withdraw from an international treaty extends to the right of those states to reverse that withdraw decision "until the moment at which the [withdrawal decision's] effects become final".
Campos Sánchez-Bordona said requiring mutual unanimous consent to revocation would be incompatible with Article 50.
"To accept that the European Council, acting by unanimity, should have the last word on the revocation of the notification of the intention to withdraw would increase the risk of the member state leaving the European Union against its will," he said. "It would suffice for one of the remaining 27 member states to block the unanimous decision of the European Council (in Article 50 format) to frustrate the will of the member state that has given notice of its desire to remain in the European Union. That state would leave (be expelled from) the European Union within two years of the notification of the intention to withdraw, against its will to remain a member of that international organisation."
The advocate general's opinion on the issue is not final. The CJEU is still to issue its judgment in the case, which the Court of Session in Edinburgh has asked the CJEU to rule on after a number of Scottish politicians posed the question to it.
The UK is scheduled to leave the EU on 29 March 2019.