Websites that link to pirated material could be in trouble for copyright infringement, the European Union’s top court has ruled.

The European Court of Justice notes in a controversial ruling (pdf) dated Sept. 8 that “when hyperlinks are posted for profit, it may be expected that the person who posted such a link should carry out the checks necessary to ensure that the work concerned is not illegally published.”

In short, for-profit websites that hyperlink to copyrighted material are on the wrong side of the law—even if they didn’t know the web link was infringing copyright—as it’s their duty to check.

The ruling concerned Playboy’s Dutch publisher, who won its legal fight to get the website GeenStijl to remove a FileFactory web link to photos of a TV celebrity, which were posted illegally. So GeenStijl was liable for the infringement of copyright, even though the material only appeared on its pages via a link to a page hosted by FileFactory.

The website slammed the ruling, describing it as a blow to press freedom. “If commercial media companies—such as GeenStijl—can no longer freely and fearlessly hyperlink it will be difficult to report on newsworthy new questions, leaked information and internal struggles and unsecure networks in large companies,” it said in a statement (link in Dutch).

While the ruling explicitly mentions a profit motive as being key, the Electronic Frontier Foundation, a digital rights group, warns the high court’s decision will affect non-profit websites, too. It noted that the definition of “financial gain” is broad enough to encompass a wide variety of websites, and that non-profit websites that hyperlink to infringing content can be liable if they knew that the material was pirated—if, for example, they receive a notice from the copyright holder.

The high court said it “accepts that it may prove difficult, in particular for individuals who wish to post such links, to ascertain whether the works involved are protected and, if necessary, whether the copyright holders of those works have consented to their publication on the internet.”