In a major blow to big internet platforms, Europe's top court said Thursday that an individual country can order Facebook to take down posts, photographs and videos and restrict global access to that material—a move that places more responsibility on internet platforms to patrol their sites for content ruled illegal.

The decision by the European Court of Justice means social networks like Facebook will be required to remove defamatory material hosted on their sites if they are aware that the content is harmful to an individual's reputation. It also means national authorities can ask social networks and other platforms to take down material that has been judged to be defamatory or "equivalent" to defamatory content.

The EU court had been asked for a ruling to clarify EU law relevant to a case referred by the Austrian supreme court that involved a Green party politician, Eva Glawischnig-Piesczek. The politician, a former leader of Austria's Green party, had sued Facebook in Ireland over content published on the company's platform that contained remarks that were harmful to her reputation.

Glawischnig-Piesczek had sought to have Facebook remove disparaging comments about her that had been posted on an individual's personal page and also to have removed "equivalent" messages posted by others. She argued that Facebook needed to delete the material in the country and limit worldwide access.

With Thursday's ruling, internet platforms such as Facebook will likely be required to take more responsibility for patrolling their sites for content that has been ruled illegal.

"This judgment raises critical questions around freedom of expression and the role that internet companies should play in monitoring, interpreting and removing speech that might be illegal in any particular country," the Facebook said in a statement issued after the ruling.

"It undermines the longstanding principle that one country does not have the right to impose its laws on speech on another country. It also opens the door to obligations being imposed on internet companies to proactively monitor content and then interpret if it is 'equivalent' to content that has been found to be illegal."

Facebook had previously warned that a ruling that allowed individual countries to force internet platforms to delete material elsewhere would limit free speech, as they would have to use automated content filters that are not highly sophisticated, and this could lead to the takedown of legitimate material.

But others had said Facebook and others needed to do more to control hate speech, defamatory posts and untrue content on the web.

The court on Thursday said that Facebook was not liable for the disparaging comments posted but it had an obligation to take down the posts after a court ruled them defamatory. Facebook, it said, "did not act expeditiously to remove or to disable access to that information."

The ruling, which came from the EU's top court, cannot be appealed.