BRUSSELS — The European Court of Justice has ruled that internet service providers cannot be forced by law to install filtering systems that prevent illegal file downloads.Decision, handed down Thursday, relates to music piracy in Belgium, but will apply across the European Union. In 2004, Belgian rights management organization SABAM obtained an injunction requiring the company Scarlet Extended to stop its customers exchanging files containing music in the SABAM repertoire. Scarlet appealed, and the Belgian court asked the European Court of Justice for advice. Its ruling concludes that, in order to filter out the offending files, the ISP would have to monitor all of its customers’ electronic communication. Such a general monitoring obligation is incompatible with European law. It also ruled that the injunction failed to respect the fundamental rights of customers and of the ISP. Requiring Scarlet to install a complicated, costly, permanent computer system at its own expense would be a serious infringement of the company’s freedom to conduct its business, it said.