European Court of Justice has 'ultimate authority' on Article 50

The most senior British member of the European Court of Justice has told Sky News that the ECJ has "ultimate authority" over Article 50, the formal process to divorce the EU.

Advocate general Eleanor Sharpston QC said Luxembourg would not interfere with the Government's Supreme Court appeal against a ruling that the Brexit process cannot begin without a parliamentary vote.

However, she said the ECJ was "fully aware of the sensitivity and delicacy and constitutional importance of the issue" - anticipating that the Supreme Court may refer it for a hearing in the European court's 28-member chamber of judges, which includes one British judge.

When asked how quickly Article 50 could be ruled on in the event of a referral, she said: "If you look at what has happened in the past when we've had an accelerated procedure, the answer is probably given within four to eight months."

:: Brexit - what happens next?

One of the Northern Irish applicants to the Supreme Court appeal has referenced the idea of a "preliminary reference" to the ECJ under Article 267 to settle the issue of whether an Article 50 notice "cannot be withdrawn once it has been given".

Asked how it was possible that one of the very institutions that the UK Government wants to leave could end up refereeing any aspect of the process to leave, Ms Sharpston told Sky News: "If you join the club and you wish to leave the club, you leave in accordance with the rules when you join the club … 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)."

She also warned that the UK legal system could "fall over" if appropriate time and care is not taken to transpose tens of thousands of European laws into national law in the Great Repeal Bill.

:: Don't use expats as Brexit 'bargaining chips', EU chief Donald Tusk warns

Ms Sharpston added: "If you want the system to work, rather than fall over, you have to spend the time and effort in working out exactly how the rights are going to work and going to be enforced. And that is, put simply, because all the law that is there at the moment is interlocking law... it is an incredibly complicated exercise - very, very complicated."

Former attorney general Dominic Grieve has warned that the transposition process could "jeopardise the rule of law".

European Free Trade Association (EFTA) court president Carl Baudenbacher also spoke to Sky News in Luxembourg.

He said he believes membership of the EFTA court is the easiest option for the UK - replacing the ECJ as arbitrator in trade disputes.

"The structure exists, the structure is tested," Mr Baudenbacher said.

However, he did suggest that other members of EFTA may not be happy with the UK using its court purely as a transitional arrangement.