Material seized from a Guardian journalist's partner after he was held at Heathrow under anti-terror laws included information that was "misappropriated" and "classified", the High Court has heard.
A senior Government intelligence adviser told judges that information taken from David Miranda - the partner of Glenn Greenwald, who has worked with US whistleblower Edward Snowden on a series of security services exposes - would harm national security if disclosed.
Oliver Robbins, deputy national security adviser for intelligence, security and resilience in the Cabinet Office, outlined why security services and police needed to "make use" of material, seized after Mr Miranda was detained earlier this month under terrorism legislation, in a statement released today by Home Office officials.
Mr Miranda, 28, who was held at the airport for nine hours on August 18, is challenging his detention in the High Court. He has launched a application for judicial review, arguing that his detention was a misuse of Schedule 7 of the Terrorism Act 2000 and breached his human rights.
His case is due to be fully aired at a High Court trial in London in October. Mr Robbins's written statement forms part of the evidence judges will analyse and was released following a preliminary hearing in London.
He said in the statement that no information so far analysed had "identified a journalistic source" or contained "items prepared by a journalist with a view to publication".
"The information that has been accessed consists entirely of misappropriated classified material in the form of approximately 58,000 highly classified UK intelligence documents," Mr Robbins added. "I can confirm that the disclosure of this information would cause harm to UK national security."
Judges have already heard that a criminal investigation is under way.
Lawyers representing police have told the court that a mass of material had been discovered, some of which was "highly sensitive" and would be "gravely injurious to public safety" if disclosed.
Mr Miranda was held without charge for the maximum time permitted under anti-terror legislation relating to port and border controls as he changed planes on a journey from Berlin to his home in Brazil.