Scarlet had been ordered by a Belgian court to filter traffic that infringed copyrights belonging to members of artists' rights agency Sabam (Société belge des auteurs compositeurs et éditeurs).
Pedro Cruz Villalón, an advocate general of the European Court of Justice (ECJ), said that such a system would violate rights guaranteed under EU law in an opinion published today.
"The installation of the filtering and blocking system is a restriction on the right to respect for the privacy of communications and the right to protection of personal data, both of which are rights protected under the Charter of Fundamental Rights," Cruz Villalón said in an opinion, according to an ECJ press release. The opinion has not yet been published in English.
Villalón said that those rights listed under the EU Charter of Fundamental Rights (21 pages / 81KB PDF), can be restricted, but only in line with national law, and only if that law meets certain standards.
"A restriction on the rights and freedoms of internet users ... would be permissible only if it were adopted on a national legal basis which was accessible, clear and predictable," Villalón said. "
SABAM, a Belgian artists and authors' rights group, won a court order in 2007 forcing ISP Scarlet Extended to create a system to block users from illegally downloading copyrighted material.
Scarlet appealed the original decision, which had given it six months to devise a system to block illegal downloads. The ISP said at the time that the ruling would force ISPs to carry out "invisible and illegal" checks on an internet users' activity.
Cruz Villalón said that any system that met the Belgian Court's demands would be extensive and would block files that no court had said definitely infringed any copyright.
"The court order would apply 'in abstracto' and as a preventive measure, which means that a finding would not first have been made that there had been an actual infringement of an intellectual property right or even that an imminent infringement was likely," he said, according to the ECJ statement.
He also said that the measure would apply to too many people because Scarlet customers would be in communication with non-Scarlet customers.
"The system must be capable of blocking any file sent by an internet user who is one of Scarlet’s customers to another internet user – who may or may not be one of Scarlet’s customers and who may or may not live in Belgium – where that file is thought to infringe a copyright managed, collected or protected by Sabam," said the statement. "It must also be capable of blocking receipt by an internet user who is one of Scarlet’s customers of any file infringing copyright which has been sent by any other internet user."
"Neither the filtering system, which is intended to be applied on a systematic, universal, permanent and perpetual basis, nor the blocking mechanism, which can be activated without any provision being made for the persons affected to challenge it or object to it, are coupled with adequate safeguards," the Advocate General said, according to the statement.
Scarlet had opposed integrating software called Audible Magic that would identify illegal content on file sharing networks, and appealed the decision to Brussels Court of Appeal.
In 2010 the Brussels Court of Appeal said it could not rule on the matter without first referring two questions to the European Court of Justice (ECJ).
Brussels has asked the ECJ to determine if delivering an injunction against ISPs forcing it to filter content suspected of copyright infringement contradicts a person's right to privacy and protection of personal data.
It also asked the ECJ if a national court should balance the extent with which it orders screening to take place with the impact it would have on those fundamental rights.
The EU's Copyright Directive says copyright owners can obtain a court order against intermediaries whose services are used for piracy. But the E-Commerce Directive says that ISPs are generally not responsible for the activity of customers and that member states must not put ISPs under any obligation to police illegal activity on its service.