LUXEMBOURG—EU competition regulators secured a big win on Thursday when Europe’s top court backed their crackdown on Belgium’s 700-million-euro ($825-million) tax scheme for Magnetrol, BP and more than 30 other multinationals. European competition chief Margrethe Vestager launched a fight against sweetheart tax deals nearly a decade ago. She has won three cases at a lower tribunal but lost two, including an order to iPhone maker Apple to pay 13 billion euros ($15.3 billion) in Irish back taxes which was dismissed by the tribunal last year. The European Union executive, the European Commission, ordered Belgium in 2016 to recover some 700 million euros from companies which benefited from the scheme. These included U.S. manufacturer Magnetrol, oil company BP, chemical producer BASF, Wabco, Cellio, Atlas Copco and Belgacom, now Proximus. The EU competition watchdog said the series of tax rulings given to the companies constituted an aid scheme. The EU Court of …