The European Court of Justice (ECJ) ruled on Thursday that forcing hosting services to monitor and filter online content is a violation of European law, dealing another blow to the controversial Anti-Counterfeiting Trade Agreement (ACTA).

The decision is of large significance after recent attempts by the entertainment and music industries to force hosting services to screen all users' communications in order to block potentially copyright-infringing content.

The ECJ ruling was issued as part of the long-running SABAM vs Netlog case. It originated in Belgian but got referred to European courts when a settlement could not be reached.

Belgian copyright collecting society, SABAM had sued the Belgium based social network Netlog, demanding the site, which qualifies as a hosting service, install a system to prevent copyright infringements carried out by its more than two million members.

"In adopting an injunction requiring the hosting service provider to install such a filtering system, the national court would not be respecting the requirement that a fair balance be struck between the right to intellectual property, on the one hand, and the freedom to conduct business, the right to protection of personal data and the freedom to receive or impart information, on the other," said the ECJ in its ruling.

Internet rights groups suggest the ruling protects internet users against regulations the entertainment industry is trying to impose on the internet.

"The decision will definitely have an impact on any attempts by the entertainment industry to go to court to impose mechanisms on internet intermediaries," said Jeremie Zimmermann, a spokesman for the French advocacy group, La Quadrature du Net (LQDN), in an interview with V3.

"The ECJ ruling proves what we having been saying for so long that private and censorship schemes are against internet users' fundamental rights."

Zimmermann said the ruling could affect the success of the ACTA, an international treaty aimed to protect the rights of copyright holders against the threat of internet piracy.