"[It] creates significant uncertainty for both U.S. and EU companies and consumers, and puts at risk the thriving transatlantic digital economy," she added.

The European Court of Justice ruled in favor of Austrian law student Max Schrems. He had complained about the way Facebook transfers his personal data to the U.S., where it can be accessed by authorities with little respect for his privacy.

He used documents exposed by the Wikileaks whistleblower Edward Snowden, and his allegations of mass spying by the U.S. intelligence services, to illustrate the problem.

In an interview with the BBC broadcast Monday, Snowden said he was ready to go to prison if U.S. authorities would accept a plea bargain.

The European court ruling is seen by some as part of a wider crackdown on big U.S. tech companies.

"It should be seen as part of a general trend - the European Court of Justice has been pro-privacy in a big way, making bold and strong decisions," said Paul Bernal, a data protection expert at the University of East Anglia.

The ruling was a "direct consequences of the Snowden revelations," he added.