The European Union's top court says key legislation allowing governments to collect data on citizens' communications for law-enforcement purposes is invalid.

The European Court of Justice in Luxembourg on Tuesday ruled the so-called data retention directive is too far-reaching and offers too few safeguards to protect people's right to privacy, creating an impression that "private lives are the subject of constant surveillance."

The legislation allows the storage of phone calls or online communication records for at least six months to help prevent serious crimes such as terrorism. The data typically reveal who was involved in the communication, when and how often, but not its content.

The court says the 2006 legislation represents a "particularly serious interference with fundamental rights."