The date was set at the end of a Court of Justice of the European Union (ECJ) hearing on Tuesday in which the world's biggest social network, technology lobbyists and governments faced off against privacy campaigners led by Schrems, who has a record of winning important cases against Facebook.

At issue are standard contractual clauses used by Facebook and other companies to transfer personal data to the United States and other parts of the world and whether these violate Europeans' fundamental right to privacy.

The contractual clauses have been used by numerous companies and other organisations especially since Safe Harbour, the European Union's previous privacy rules, were struck down in a previous case brought by Schrems.

Cross-border transfers worth billions of dollars are a fact of life for businesses ranging from banks to carmakers to industrial giants.

The ECJ Advocate General's opinion will be non-binding but such opinions are influential and usually followed by the court's judges at a later date.