Britain's High Court has ruled that material seized from the partner of a journalist working to publish secrets from US leaker Edward Snowden can be partially examined by police.
UK Police launched a criminal investigation over the data, claiming the files it has seen are "highly sensitive" and would be "gravely injurious to public safety" if revealed.
David Miranda, 28, the partner and assistant of Guardian journalist Glenn Greenwald, was detained for nine hours at London Heathrow Airport under anti-terror laws as he changed planes on Sunday.
The Brazilian, who helped Greenwald work on the Snowden material, had his laptop, phone, memory cards and other electronic equipment confiscated by agents.
Lawyers for Miranda asked the High Court to prevent the government from "inspecting, copying or sharing" the data.
Instead, the court decided to allow the government to view the items on the condition the material was being examined on "national security" grounds.
Calling Thursday's ruling a partial victory, Miranda's lawyer Gwendolen Morgan said the Home Office and London police headquarters Scotland Yard now had seven days to prove there was a genuine threat to Britain's security.
"The defendants are not to inspect, copy, disclose, transfer or distribute - whether domestically or to any foreign government or agency - or interfere with the materials obtained from Mr Miranda under Schedule 7 of the Terrorism Act, save for the purposes of protection of national security," she said outside court.
"The very chilling effect of the implications of today's judgement are something that journalists worldwide should be very concerned about."
Snowden, a former US National Security Agency contractor, leaked information on mass surveillance programs conducted by the NSA and Britain's electronic eavesdropping agency GCHQ (Government Communications Headquarters).
Based on the material Snowden provided, British newspaper the Guardian has published a series of reports detailing the programs.
A lawyer for London's Metropolitan Police, Jonathan Laidlaw, told the court that a mass of data had been discovered by officers, who were still examining the material.
"That which has been inspected contains, in the view of the police, highly sensitive material disclosure of which would be gravely injurious to public safety," he said.
Home Secretary Theresa May said before the hearing that the police were right to act if they thought Miranda was carrying material for Greenwald that could be useful to terrorists.
Separately, David Anderson, Britain's independent reviewer of terror legislation, said Thursday he would launch an investigation to consider whether the anti-terror laws used to detain Miranda were "lawfully, appropriately and humanely used".
