Facebook can be ordered to remove illegal content worldwide by national courts, Europe's top court has ruled.
The judgment means social platforms can be forced to seek out hateful content deemed illegal by a national court in the 28-country bloc, rather than wait for requests to remove posts as it currently does under EU rules.
Facebook and other platforms can also be made to comply with requests to take down content globally, even in countries where it is not illegal, the Luxembourg-based Court of Justice of the European Union (CJEU) ruled.
The judgment comes a week after the same court told Google that it does not have to apply Europe's "right to be forgotten" law globally, garnering praise from freedom of speech advocates as courts try and figure out how much responsibility for content platforms should take.
"EU law does not preclude a host provider like Facebook from being ordered to remove identical and, in certain circumstances, equivalent comments previously declared to be illegal," the Court said in a statement.
"In addition, EU law does not preclude such an injunction from producing effects worldwide, within the framework of the relevant international law."
Facebook slammed the ruling, saying that it was not the role of social platforms to monitor, interpret and remove speech that may be illegal in any particular country.
"It undermines the long-standing 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," the company said.
"In order to get these right national courts will have to set out very clear definitions on what 'identical' and 'equivalent' means in practice. We hope the courts take a proportionate and measured approach, to avoid having a chilling effect on freedom of expression."
UK rights group Article 19 backed Facebook, saying it could lead to social platforms installing automated filters to screen out content.
"This would set a dangerous precedent where the courts of one country can control what internet users in another country can see. This could be open to abuse, particularly by regimes with weak human rights records," executive director Thomas Hughes said.
This comes after Eva Glawisching-Piesczek, then chairwoman of the Greens parliamentary group in Austria, sued Facebook in an Austrian court, asking that it delete defamatory comments posted by a user and also to remove equivalent claims.
Ms Glawischnig's lawyer welcomed the judgment.
"The CJEU is taking a position on this issue in that it is saying one must create and strengthen instruments for the enforcement of personal rights," she told a Austrian national broadcaster.
"It is less about there being fewer hate postings (after the ruling) but much more about increasing awareness that we can successfully defend ourselves against this and that it will be used more and that this then strengthens all of us in the fight against online hate speech."
The European Commission said the ruling was limited to court orders and doesn't apply to wider complaints by users alleging that certain content is illegal.
The court's ruling is the final judgment in case C-18/18 Eva Glawischnig-Piesczek v Facebook Ireland Limited.