Europe's top court is to deliver its verdict on an agreement that gave US spy chiefs access to the online data of millions of citizens.
Austrian privacy campaigner Max Schrems will learn if he has shattered the Safe Harbour treaty in his fight to expose what information Facebook gave to American intelligence agencies.
The European Court of Justice will issue its final ruling less than a fortnight after its legal officer advised that the agreement was invalid and amounted to "mass, indiscriminate surveillance".
The ramifications could be immense, Mr Schrems's Irish lawyer said.
Not only are European Union officials trying to rewrite the agreement but a decision in favour of online privacy could further strain EU-US relations already damaged by the revelations of spy whistleblower Edward Snowden.
The Luxembourg-based court does not have to follow the advice it receives from its Advocate General, the ECJ legal officer, but more often than not it does.
Gerard Rudden, Mr Schrems's lawyer, said there is also the potential for lawsuits over Safe Harbour.
"If it is held that Safe Harbour is invalid and Facebook have been transferring data, that could open the door to compensation claims," he said.
Mr Schrems's legal battle over Safe Harbour was sparked by Mr Snowden's revelations over the US National Security Agency (NSA)'s Prism surveillance system which allowed spies to access enormous amounts of data from global tech companies.
He initially took a lawsuit in Ireland after failing to secure an investigation into Facebook by the country's Data Protection Commission, which has the authority to audit the social media giant.
Mr Schrems claimed Ireland's data watchdog had an onus to uncover what information Facebook held on users and ultimately what was being transferred to the US under Safe Harbour and being accessed through Prism.
The case was taken in Dublin as every Facebook user outside the US and Canada has a contract with Facebook Ireland.
The ECJ judgment will be sent to the High Court in Dublin where the judge will use it as the basis for deciding on Mr Schrems's legal challenge for Facebook to be audited.
Mr Snowden, a former NSA contractor now in exile in Russia, triggered a wave of controversy when he leaked tens of thousands of documents about surveillance programmes run by the US intelligence services and foreign counterparts, including Britain's GCHQ, in 2013.
He fled to Hong Kong where he met journalists to co-ordinate a series of articles that exposed mass surveillance programmes such as the NSA's Prism and GCHQ's Tempora, which involve "hoovering up" vast volumes of private communications.