Documents seized from the partner of Guardian journalist Glenn Greenwald might threaten Britain's national security, damage the economy and lead to "widespread loss of life", a government adviser said.
The information, taken from David Miranda on August 18 at Heathrow Airport, "is highly likely to describe techniques which have been crucial in life-saving counter-terrorist operations" and could identify British intelligence agents abroad, Oliver Robbins, a UK national security adviser, said in documents released during a court hearing.
It was the first time the government had offered specific reasoning behind why security services and police are so concerned about material seized from Miranda.
Greenwald has written stories based on material leaked by former US National Security Agency contractor Edward Snowden.
Robbins also said that Miranda and others had shown "very poor judgement in their security arrangements", including carrying a piece of paper containing the password for encrypted files.
He said media stories about the documents seized from Miranda had already caused harm.
Guardian editor-in-chief Alan Rusbridger dismissed the statement as containing "unsubstantiated and inaccurate claims".
Miranda, who returned to Brazil after being released by officers, had asked the London court to issue an injunction stopping police from examining the material.
But the court heard yesterday that lawyers representing him and the British authorities had agreed to give British police and the interior ministry until October - when a review will be held - to continue examining the seized data.
London's Metropolitan Police Service welcomed the court decision, saying in a statement that it needed to sift through the highly sensitive material to protect public safety.
Miranda's attorneys described the decision as a pragmatic one and said that Miranda had decided he would make his full argument in October.
Bloomberg, Reuters, Associated Press