The UK Home Office says it has "to protect the public", but Miranda has accused Britain of a "total abuse of power" and has said he will take legal action against the Home Office. The Guardian is "supportive" of his action.
David Miranda, the partner of US journalist Glenn Greenwald, who published documents about the NSA and GCHQ spying activities leaked by Edward Snowden, in Britain's Guardian newspaper was questioned for nine hours in London's Heathrow airport on Sunday under Schedule 7 of the UK's anti-terrorism law.
Miranda's lawyer, Gwendolen Morgan, said her client was seeking a judicial review of the legal basis for his detention and wanted assurances that the property seized from him by police would not be examined.
"We've sought undertakings that there will be no inspection, copying or disclosure, transfer or interference in any other way with our client's data," she told Reuters.
Morgan said the "letter before action" had been sent to London's police chief and the Home Secretary. It also demanded that they detail whether Miranda's data had already been passed on to anyone else, and if so, to whom and why.
"We're waiting to hear back this afternoon from both the defendants. Failing that we will be left with no option but to issue urgent proceedings in the High Court tomorrow," she said.
It is unclear whether this would in reality stop British authorities from inspecting the data, which is already in their possession. But the Guardian has said that many journalists pass through Heathrow every day and should not have to worry about whether their material is safe. The paper has said it is "supportive" of Miranda.
The letter form Miranda's lawyer to the UK's Home Office, as published by the Guardian."Total abuse of power"
Miranda, in his first interview since arriving home in Rio de Janeiro, said the UK authorities were trying to intimidate him and threatened him with jail unless he cooperated and handed over the passwords to his computer and mobile phone.
"They were threatening me all the time and saying I would be put in jail if I didn't co-operate. They treated me like I was criminal or someone about to attack the UK. It was exhausting and frustrating but I knew I wasn't doing anything wrong," he said.
Miranda was on his way back to Brazil from Berlin where he had been ferrying material back for his partner Greenwald and Laura Poitras, a filmmaker who has also been working on stories related to the NSA files leaked by Edward Snowden.
"I was in a country with different laws, in a room with seven agents coming and going who kept asking me questions. I thought I might be detained for a very long time," he continued.
"This law shouldn't be given to police officers. They use it to get access to documents or people that they cannot get the legal way through courts or judges. It's a total abuse of power," he added.
Miranda says he was offered a cup of water and was offered a lawyer but says he refused both and didn't have a drink until he got a Coke from the machine in the corridor after eight hours of questioning.