A privacy campaigner has scored a legal victory that could bolster his attempts to prevent Facebook from being able to pass EUcitizens' data to the US authorities.
An opinion issued by the European Court of Justice says that current data-sharing rules between the 28-nation bloc and the US are "invalid".
The decision could affect other tech firms' abilities to send Europeans' information to US data centres.
However, it is not a final judgement.
Although the EU's highest court tends to follow the opinions of its legal adviser, the 15 judges involved have yet to issue a conclusive ruling of their own on the matter.
Even so, Max Schrems - the activist who prompted the case - suggests there could be far-reaching consequences.
"Companies that participate in US mass surveillance and provide, for example, cloud services within the EU and rely on data centres in the US may now have to invest in secure data centres within the European Union," he said.
"This could be a major issue for Apple, Facebook, Google, Microsoft or Yahoo.
A spokeswoman for the social network said: "Facebook operates in compliance with EU Data Protection law. Like the thousands of other companies who operate data transfers across the Atlantic we await the full judgement."
Safe harbourThe origins of Mr Schrems' dispute with Facebook can be traced back to whistleblower Edward Snowden's leaks about US cyberspies' activities.
In 2013, Snowden released details about a surveillance scheme operated by the NSA called Prism, which provided officials with ways to scrutinise data held by US tech firms about Europeans and other foreign citizens.
Mr Schrems alleged that, in light of the revelations, EU citizens had no protection against US surveillance efforts once their data had been transferred.
He targeted Facebook in particular because of the wide range of data it gathered and the number of people using it.
However, when he took the case to Ireland - where Facebook's European headquarters are based - it was initially rejected.
The Irish data watchdog said the Safe Harbour agreement between the US and EU prevented it from intervening.
When Mr Schrems challenged the watchdog in the Irish courts, the matter was referred to the European Court of Justice.