Material seized from a Guardian journalist's partner after he was held at Heathrow under anti-terror laws would harm national security if disclosed, 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 - included information that was "misappropriated" and "classified".
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 by Home Office officials.
But following a hearing in London, Alan Rusbridger, editor-in-chief of Guardian News and Media, said: "Mr Robbins makes a number of unsubstantiated and inaccurate claims in his witness statement. The way the Government has behaved over the past three months belies the picture of urgency and crisis they have painted.
"The Government claims that they have at all times acted with the utmost urgency because of what they believed to be a grave threat to national security. However, their behaviour since early June - when the Guardian's first Snowden articles were published - belies these claims."
Mr Miranda, 28, who was held at the airport for nine hours on Sunday August 18, is challenging his detention in the High Court. He has launched an 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 October.
Mr Robbins's written statement was released following a preliminary hearing when Lord Justice Laws and Mr Justice Kenneth Parker heard that the parties involved had agreed on the terms of a temporary order relating to the extent of the use of the material taken from Mr Miranda pending that hearing.
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.