ISPs cannot be legally obliged to monitor their customers' electronic communications and block the unauthorised transmission of copyrighted content, the European Court of Justice (ECJ) has ruled, in a landmark decision that will come as a blow to rights holders.

The ruling came in the Scarlet Extended case, which began life in the Belgian courts in 2004 and was escalated to the ECJ.

Belgian rights holders group SABAM was looking to impose content filtering requirements on ISP Scarlet to prevent illegal downloading and distribution in the country.

"EU law precludes the imposition of an injunction by a national court which requires an internet service provider to install a filtering system with a view to preventing the illegal downloading of files," the court ruled on Thursday.

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