The EU’s top court should back a ruling that overturned regulatory approval for Sony Corp. and Bertelsmann AG’s 2004 deal to combine their music units and form the world’s second-largest record label, a legal adviser to the EU’s highest court said Thursday.

In a legal opinion that is not binding on the European Court of Justice but is followed in most cases, Advocate General Juliane Kokott said the court should uphold a decision made last year by the Court of First Instance that stripped away EU clearance to a business that was already up and running.