The High Court ruled authorities can only partially examine material seized from David Miranda, who was detained for nine hours Sunday at Heathrow airport under anti-terror laws, the BBC reported.
Attorneys for Miranda, the partner of The Guardian journalist Glenn Greenwald, who exposed the massive monitoring system used by the National Security Agency, filed for the injunction, which prevents the British government and police "inspecting, copying or sharing" the seized data, except for the purposes of national security.
The Home Office told the court "tens of thousands of highly classified documents" were seized, the BBC said.
Miranda was detained while traveling from Berlin to Rio de Janeiro, where he lives with Greenwald. Miranda said he was threatened with prison if he didn't cooperate.
Greenwald reported on U.S. and British surveillance programs for The Guardian based on leaks from former NSA contractor Edward Snowden, who was granted temporary asylum in Russia.
In a letter to the Home Office, Thorbjorn Jagland, head of the European human rights watchdog Council of Europe, expressed concern about Miranda's treatment, warning it "may have a potentially chilling effect on journalists' freedom of expression."
Miranda's lawyers said nine items, including his laptop, mobile phone and DVDs, were seized during the detention.
His lawyers said they began legal proceedings to "protect the confidentiality of the sensitive journalistic material" and that their motion was meant to secure a temporary injunction to prevent the authorities using the material.
Brazilian Foreign Minister Antonio Patriota said his country would not retaliate against Britain over the incident but was awaiting an explanation.
Patriota said he told British Foreign Secretary William Hague in a telephone conversation holding Miranda was "counterproductive and does not contribute to an international coordinated action," the BBC said.
Home Secretary Theresa May has defended police use of anti-terrorism laws to hold and question Miranda, saying such action "was right" if police believed Miranda had information useful to terrorists.