Brexit legal challenge: European Court of Justice has 'ultimate authority' on Article 50, British member says

'If you join the club and you wish to leave the club, you leave in accordance with the rules'

Pro-Brexit protest in London on 23 November (Getty Images)

The European Court of Justice has “ultimate authority” on Brexit, its most senior British member has warned.

Advocate general Eleanor Sharpston QC told Sky News judges hearing the Government's upcoming Supreme Court appeal against a ruling that Article 50 cannot be triggered without a Parliamentary vote could refer the matter to Luxembourg.

She said the 28-member ECJ was “fully aware of the sensitivity and delicacy and constitutional importance of the issue”, adding that a European ruling could take between four and eight months in the event of a referral.

Any input from Strasbourg would be certain to inflame hardline Brexit supporters, after the Prime Minister vowed to end its jurisdiction in the UK.

“If you join the club and you wish to leave the club, you leave in accordance with the rules when you joined the club," Ms Sharpston added. "The rules of this club are the ones contained in Article 50, and the interpretation of those rules is a matter for this Court (the ECJ).”

A law professor has already warned that Theresa May is heading for an 11-0 defeat when Supreme Court justices rule on whether Parliament must approve starting the formal process for leaving the European Union.

Professor Michael Zander QC said the High Court judges who ruled the Prime Minister could not act alone when triggering the Article 50 notice had given a “unanimous and very strong” decision.

The ECJ's president predicted the matter may end up before his judges earlier this month.

Koen Lenaerts, Europe’s most senior judge, said there are “many, many ways” Britain’s departure from the EU could end up before the EU’s highest court.

“I can’t even start, intellectually, imagining how and where and from which angle it might come,” he added.

Mr Lenaerts suggested there were numerous unforeseen legal consequences of the exit process that the EU’s top court may yet be called on to resolve.

They included the treatment of Article 50 and possible constraints on the sort of post-Brexit agreement that the EU’s other countries might seek.