Allowing the UK to unilaterally pull out of Brexit would destabilise the EU and lead to “disaster”, lawyers acting on behalf of Brussels have said.
The claim was made during a one-day hearing in a landmark case that will see Europe’s highest court decide whether Article 50, the legal device to trigger an exit from the EU, can be unilaterally revoked.
The case, which was heard on Tuesday at the European Court of Justice in Luxembourg, is being brought by a cross-party group Scottish politicians who say MPs should know all their options before voting on Theresa May’s Brexit deal in December.
Aidan O'Neill QC, representing the petitioners, said it was "fundamental" to the treaties of the EU that an individual member state can unilaterally revoke the decision to withdraw.
He said: "It cannot be in the interest of the union as a whole to force a member state to leave the union against the wishes of the people.
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"The union's wider interest lies with member states remaining in the EU when their peoples wish to do so."
Mr O’Neill added that the politicians who brought the case "need to know the options for revocation, which are open to withdrawing member states now, to allow them, properly and in a fully informed way, to carry out their duties as democratically elected representatives, accountable to the people."
Advocate General for Scotland Lord Keen QC, representing the UK Government, said London did not have a position on the central question in the case, instead arguing that it should be ruled inadmissible as it is a "hypothetical validity challenge".
Lord Keen asked the court not to open "this Pandora's box" and said the UK Government does not intend to revoke Article 50. He claimed the petitions were seeking "political ammunition to be used in and to pressure the UK Parliament".
Hubert Legal, representing the Council of the European Union, argued that if unilateral withdrawal was allowed, "the main victim could be the European project altogether".
He said Article 50 is "not ambiguous", adding: "The prerogative of acting alone will have been exhausted by putting the notification letter on the council's table."
Mr Legal added that if EU member states could invoke and withdraw Article 50 unilaterally, they would use the power as leverage to demand special terms from Brussels.
He claimed it would “encourage notifications that could not be, if not frivolous, at least tentative or conditional, and inspired by the hope to obtain a better deal in relations with Union partners than what regular membership offers.”
Under those circumstances, Mr Legal argued that the two-year Article 50 negotiation period would “become a pseudo-convention, an open forum of good intentions, meant to charm the notifying sheep back to the flock through promises of differentiated treatment”.
Mr O'Neill accused EU institutions of inviting the court to act "unconstitutionally and in contravention of the rule of law by reinterpreting the treaties".
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The ECJ is the highest court in Europe regarding EU law and a written judgment on the case is expected in the coming weeks, with the president of EJC saying it will happen "quickly".
Scotland's most senior judge Lord Carloway ruled in September to refer the question to the ECJ after the case was heard at the Court of Session in Edinburgh, Scotland's highest civil court.
UK Government attempts to appeal against this ruling were rejected by the Court of Session and the Supreme Court.
The case has been brought by Labour MEPs Catherine Stihler and David Martin, Joanna Cherry MP and Alyn Smith MSP of the SNP, and Green MSPs Andy Wightman and Ross Greer, together with lawyer Jolyon Maugham QC, director of the Good Law Project.