(CNN) -- Data seized from the partner of journalist Glenn Greenwald, who has led The Guardian newspaper's reporting on secret U.S. surveillance programs, would cause harm to UK national security and could expose British spies to risk if it is disclosed, London's High Court heard Friday.
David Miranda filed a legal challenge against UK authorities last week after they detained him as he passed through London's Heathrow Airport on his way home to Brazil and confiscated items including a laptop, hard drive and USB memory sticks from him.
Miranda won a partial victory in his bid to prevent UK authorities examining, copying or sharing the material seized while he challenged the legality of his detention under terrorism legislation, but the judge left a loophole that allows the police to continue investigating the materials they seized from him for the purpose of protecting national security.
The government argues it had a duty to detain him and confiscate the material to ensure national security.
Miranda's partner, Greenwald, has been at the forefront of high-profile reports exposing hidden U.S. intelligence programs, based on leaks from former U.S. National Security Agency contractor Edward Snowden.
The information accessed so far includes "misappropriated classified material" made up of about 58,000 highly classified UK intelligence documents, Oliver Robbins, Britain's Deputy National Security Adviser for Intelligence, Security and Resilience, said Friday in a written statement to the court.
No information analyzed so far "has identified a journalist source or has contained any items prepared by a journalist with a view to publication," he said.
Robbins said the material seized was "highly likely to describe techniques which have been crucial in life-saving counter terrorist operations"and other intelligence activities," and that any disclosure of them would jeopardize national security and aid those hostile to Britain.
"A particular concern for HMG (the UK government) is the possibility that the identity of a UK intelligence officer might be revealed. It is known that contained in the seized material is personal information that would allow staff to be identified, including those deployed overseas," he said.
"Real damage" has already been done to UK national security by media revelations resulting from the leaked material, he said.
While much of the data was encrypted, one of the paper documents taken from Miranda explained how to decrypt some of it, Robbins said.
He said this was a sign of "very poor information security practice," and that possession of the information by those outside the intelligence services put it at risk of being accessed by hostile elements.
The volume of data found in Miranda's possession suggests that Snowden "scraped" data from classified networks in bulk and passed it on, Robbins said, rather than undertaking targeted and careful appropriation of documents.
A large proportion of the material is classified as "Secret" or "Top Secret," he said.
He argued that UK authorities may need to make more copies of the confiscated files in order to decode those that are still encrypted and to uncover London's Metropolitan Police have also launched a criminal investigation in the case.
Law firm Bindmans, which is representing Miranda, said the Home Office and police had made "sweeping assertions about national security threats" in their court filing but given no details to back up the claim.
Miranda "does not accept the assertions they have made and is disappointed that the UK Government is attempting to justify the use of terrorist powers by making what appear to be unfounded assertions," a statement from the law firm said.
The statement quotes Miranda as saying his rights have been violated and "that basic press freedoms are now threatened by the attempted criminalization of legitimate journalistic work."
Miranda was stopped at Heathrow in mid-August as he was returning to the couple's Rio de Janeiro home after staying in Berlin with filmmaker Laura Poitras, who has been working with Greenwald on NSA-related stories.
Miranda has said he doesn't know what material he was carrying. He doesn't work for The Guardian, but the newspaper paid for his flights because he was helping his partner.
He and Greenwald told CNN's Anderson Cooper of their distress and anger about his treatment at Heathrow Airport.
"To start detaining people who they think they are reporting on what they're doing under terrorism laws, that is as dangerous and oppressive as it gets," said Greenwald.
CNN's Susannah Palk contributed to this report.