British MPs could 'unilaterally' halt Brexit

His "opinion" was "non-binding" on EU judges, but most verdicts do follow the advocates' ideas, an EU court contact told EUobserver.

The court has also promised a "quick" ruling in the case, as British MPs prepare to vote on the UK-EU withdrawal agreement in the House of Commons on 11 December.

Scottish MPs and MEPs had brought the case on Article 50 - a 250-word clause inserted into the EU charter eight years ago, which had never been used until the UK voted to exit the European Union in the summer of 2016.

British government lawyers argued in an EU court hearing last month that the case was inadmissible because the UK had no intention to revoke its decision.

But that was not true, given the House of Commons vote, the advocate general said.

"The Court of Justice's answer will have the effect of clarifying the precise options open to MPs when casting their votes," Campos Sanchez-Bordona said.

"The dispute is genuine, the question is not merely academic, nor premature or superfluous, but has obvious practical importance," he added.

Lawyers for the EU Council and the European Commission also argued that Article 50 could only be reversed by a unanimous decision of all 28 EU capitals.

To allow unilateral revocation would open a "Pandora's box", they said, in which member states feigned to leave in order to get opt-outs or other perks in the exit talks, before making a U-turn.

But unanimous revocation could "in practice entail the forced exit from the EU of a state which ... continues to be an EU member state in all respects," the advocate general said.

"It would be illogical to force that member state to withdraw from the EU in order to then have to negotiate its accession," he said.

It would also undermine the "sovereignty" of the UK to make decisions on its own future and could create a situation in which Brexit had no constitutional validity in Britain but still went ahead.

Forcing someone out would affect "the rights acquired by EU citizens, which the withdrawal of a member state will inevitably restrict," he added.

'Abusive practices'

The opinion imposed conditions to prevent a Pandora scenario, however.

Unilateral revocation must "not involve an abusive practice" and "principles of good faith and sincere cooperation must ... be observed", Campos Sanchez-Bordona said.

It can only take place in the two-year period from the member state's formal notification of triggering Article 50 to the EU Council, where its peers meet, he added.

And it can happen only if the withdrawal treaty has not yet been "formally concluded", if the revocation "respect[s] national constitutional requirements", such as a parliament vote, and is "formally notified" to the EU Council, he said.

The jurist also zoomed in on the language of Article 50.

"To withdraw is to notify the European Council of 'its intention' - and not of its decision - to withdraw, and such an intention may change," he noted.

The court opinion was welcomed by Jo Maugham, a lawyer on the Scottish plaintiffs' side, who told the Financial Times, a British newspaper, that it "puts the decision about our future back into the hands of our own representatives - where it belongs".

But it was attacked by pro-Brexit hardliners, such as British eurosceptic MEP Nigel Farage, who said: "Every effort is being made on both sides of the [English] Channel to stop Brexit".

British MPs on alert

The opinion came out the same day British prime minister Theresa May was to open a five-day long Brexit debate in Westminster.

"The British people want us to get on with a deal that honours the referendum and allows us to come together again as a country, whichever way we voted," she plans to tell MPs, according to excerpts of her speech shared with media.

"I will still have a job in two weeks' time," no matter how MPs vote on 11 December, she told ITV, a British TV broadcaster, on Tuesday morning, amid speculation May would resign if she lost the vote.

"My job is making sure that we do what the public asked us to: We leave the EU but we do it in a way that's good for them," she said.

Geoffrey Cox, her attorney general, also backed the withdrawal accord in remarks to MPs on Monday.

"This is the deal that will ensure that happening [exiting the EU on 29 March 2019] in an orderly way with legal certainty," he said.

But Cox himself faces MPs' calls to cite him for contempt of parliament, on grounds that he refused to publish a legal opinion on how May's withdrawal deal would impact the United Kingdom.

The opinion is sensitive given the withdrawal accord's special provision on keeping the UK in the EU customs union until such time as it finds a way to leave without imposing new border checks on the Irish land border.