Advocate general Eleanor Sharpston QC told Sky News the Luxembourg court would not actively interfere with the Government’s Supreme Court appeal but warned the case could be referred to the 28-member chamber of judges.
The appeal comes after the High Court ruled that there had to be a parliamentary vote to trigger Article 50 following a legal challenge by campaigners.
Ms Sharpston said the ECJ was “fully aware of the sensitivity and delicacy and constitutional importance of the issue” and said a referral could put Brexit on hold for months.
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.”
The interview comes after a Northern Irish applicant to the Supreme Court appeal proposed a “preliminary reference” under Article 267 to settle the issue of whether an Article 50 notice can or cannot be withdrawn once it has been filed.
Addressing concerns of the EU overseeing aspects of the process of leaving the bloc, she said: “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).”
Ms Sharpston also issued a stark warning that the UK legal system could “fall over” if time was not taken to properly transpose tens of thousands of European laws into national law in the Great Repeal Bill.
“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,” she said.
“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.”
The comments come after former attorney general Dominic Grieve warned that transposition could “jeopardise” the rule of law.