Judges ask government to provide detailed evidence about why it wants right to trawl data seized using terror laws

The high court has given the government until Tuesday night to provide detailed evidence about why it wants the right to trawl and share data seized using terror laws from the partner of a Guardian journalist.

Lord Justice Beatson and Judge Kenneth Parker said in a judgment outlining their decision to allow the police to continue accessing material taken from David Miranda that the ruling was made because while they could not judge the strength of the government's claims about the national security risks the material would pose if disclosed, they did have "serious assertions by responsible persons".

Miranda's partner, Glenn Greenwald, has exposed mass digital surveillance by US and UK spy agencies based on material leaked by Edward Snowden, the former US intelligence contractor. Miranda was travelling from Berlin back to his home in Rio de Janeiro when he was detained at Heathrow last Sunday.

Lawyers for the government and Metropolitan police have so far claimed the data seized from Miranda included "highly sensitive material" and "tens of thousands of highly classified UK intelligence documents, the unauthorised disclosure of which would threaten national security, including putting lives at risk". But detail has been limited.

The judges said the lack of further evidence from the authorities was understandable given the rapidly moving case, but it had been "a difficulty". They said the protection of journalistic sources and the protection of national security were competing interests, but "the public interest in the investigation, detection and prosecution of those who are reasonably suspected to be terrorists" justified their decision to allow police to retain the material.

On Thursday, the judges had ruled that the police could retain Miranda's data until next Friday, but added that they are only allowed to examine the data in the context of the protection of national security or for investigating if Miranda himself was involved in terrorism. The judges also said that the data could not be used in the context of a criminal investigation.

It also emerged on Thursday that the Met launched a criminal investigation on Thursday, shortly before Miranda's application for restrictions on the use of the data was first heard in court.

Early on Friday police indicated to Miranda's lawyers that they were prepared to hand back items seized from him last weekend. These were to include DVDs, two watches and a laptop. But on Friday night the police indicated they would not be releasing the items yet.

A letter from the Treasury Solicitor to the court claiming a threat to national security from any unauthorised disclosure of the data said: "It is not possible in this letter to give more of the particulars of this assertion. But we make it on instructions and after having taken advice from the relevant person.".

A full hearing is due to take place next Friday, which will establish how the government is able to use the data in the longer term.

The detailed judgment also gave further clues about what the UK authorities may be doing with the data. Government lawyers have told the court that under the Counter-Terrorism Act 2008 "once material has been lawfully obtained pursuant to schedule 7, it may be disclosed to intelligence services who may then use it for their statutory purposes".

Liberty, the civil liberties and human rights group, has applied to formally intervene in the planned judicial review this autumn into the legality of Miranda's detention and the seizure of his computer hardware. In a letter sent to the court, it said it was "a matter of grave concern that the power has now been directly targeted at the close family member of a prominent journalist in a manner that would appear to be a direct attempt to interfere with press freedom".

Joel Simon, executive director of the New York-based Committee for the Protection of Journalists this week wrote to David Cameron complaining Miranda's detention was "not in keeping with the UK's historic commitment to press freedom".

Thorbjorn Jagland, secretary general of the Council of Europe, has written to the home secretary Theresa May about the detention of Miranda and the destruction of Guardian computers at the request of the UK authorities.

He said that if confirmed, "these measures … may have a potentially chilling effect on journalists' freedom of expression as guaranteed by article 10 of the European convention on human rights". He asked May to "comment on the compatibility of the measures taken with the UK's obligations under the convention".