France's highest constitutional authority said taking information during searches conducted under state of emergency measures constitutes unauthorized seizure, the council said in a decision released Friday.
Copying and using data stored on computers belonging to those being searched was unconstitutional because it did not safeguard a fair balance between preserving public order and respecting privacy, the council said.
Police will be required to destroy data obtained during such searches, according to French newspaper Le Monde. The council did not say, however, that reviewing information during a search was unconstitutional.
Other measures passed under the state of emergency, including the right to close theatres or meeting places, were also not considered to be necessarily unconstitutional in a parallel decision by the council.
The council made the ruling in response to a complaint brought by France's Human Rights League, one of the groups that has opposed measures imposed under the state of emergency.
The league's lawyer Patrice Spinosi told Le Monde that the decision "demonstrated the legitimacy of our action, even if the victories are marginal."
France's parliament passed the state of emergency after terrorist attacks in Paris on November 13 left 130 people dead and 350 injured. It has been extended twice, and will remain in effect until May.
The government is pushing for an amendment that will enshrine states of emergency in the country's constitution.