David Miranda is challenging the legality of his detention at Heathrow Airport, his lawyers have said.
Solicitors for Miranda, partner of Guardian journalist Glenn Greenwald, have written to the Home Secretary and Met Police commissioner for assurances that "there will be no inspection, copying, disclosure, transfer, distribution or interference, in any way, with our client's data pending determination of our client's claim", law firm Bindmans LLP said.
Miranda was detained at Heathrow Airport under Schedule 7 of the Terrorism Act 2000 as he changed planes on a journey from Berlin to his home in Brazil.
The announcement of his legal action comes as a Downing Street source denied any political involvement in the decision to detain him.
A Downing Street source said Number 10 was "kept abreast of the operation in the usual way" but denied any political involvement in the decision to detain Mr Miranda.
"The Government does not direct police investigations," they said.
The statement comes after shadow home secretary Yvette Cooper called on the Home Secretary to say whether she or the Prime Minister was informed about the decision, after a White House spokesman said it was given a "heads up" on the move to detain Miranda.
In a statement released today, Bindmans, which is representing Miranda, said it is challenging the "legality of the action" under Schedule 7 of the Terrorism Act.
Criminal lawyer Kate Goold said: "We are most concerned about the unlawful way in which these powers were used and the chilling effect this will have on freedom of expression."
The firm said that its solicitor Gwendolen Morgan, who is representing Miranda, has written to the Home Secretary and the Met Commissioner calling for assurances that "there will be no inspection, copying, disclosure, transfer, distribution or interference, in any way, with our client's data pending determination of our client's claim.
"We require undertakings that the product of that inspection or interference will not be disclosed, shared or used further in any way, and will be kept secure pending the outcome of our client's challenge to the legality of the seizure of that data," Ms Goold said.
"If any other public authority or third party – either domestic or foreign – has been granted possession or access to that data (or copies of it) we require you to provide similar undertakings from each of those parties if you are in a position to do so.
"Further, or alternatively, we require immediate disclosure by you of the identity of those parties to whom such access or disclosure has already been given so that we may obtain similar undertakings from them directly."
Miranda was detained at Heathrow Airport under Schedule 7 of the Terrorism Act 2000 as he changed planes on a journey from Berlin to his home in Brazil.
He claimed he was held for nine hours by agents who questioned him about his "entire life" and took his "computer, video game, mobile phone, my memory card – everything".
Schedule 7 applies only at airports, ports and border areas, allowing officers to stop, search, question and detain individuals.
Its use has been criticised by Mr Greenwald – the reporter who interviewed American whistleblower Edward Snowden – as a "profound attack on press freedoms and the news-gathering process", and has sparked widespread debate on the use of terror laws.
A Guardian News & Media spokesman today said the Guardian is "supportive" of Miranda's claim.
The newspaper's editor Alan Rusbridger told the BBC: "We will have to work out if that is legal or not, and that will be subject to legal challenge, I believe, by Glenn Greenwald and David Miranda because it's not clear that he was actually committing any offence in carrying material through Heathrow.
"I'm not aware of what that offence is."
He went on: "I think there will be a legal challenge to the use of this very controversial bit, this Schedule 7, of the Terrorism Act and its use to obtain journalistic material.
"A lot of journalists the world over fly through Heathrow and I think some of them now are going to be quite anxious about how the British authorities regard this bit of Britain that is not quite Britain but is Britain."
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