The European judges concluded:

The Court takes the view that, by requiring the retention of those data and by allowing the competent national authorities to access those data, the directive interferes in a particularly serious manner with the fundamental rights to respect for private life and to the protection of personal data. Furthermore, the fact that data are retained and subsequently used without the subscriber or registered user being informed is likely to generate in the persons concerned a feeling that their private lives are the subject of constant surveillance.

. . .

Although the retention of data required by the directive may be considered to be appropriate for attaining the objective pursued by it, the wide-ranging and particularly serious interference of the directive with the fundamental rights at issue is not sufficiently circumscribed to ensure that that interference is actually limited to what is strictly necessary.