The boyfriend of an investigative journalist who was held under UK terror laws has started legal action over his detention.
David Miranda has appointed law firm Bindmans LLP to pursue a civil action over his treatment at Heathrow Airport on Sunday.
The solicitors have written to the Home Secretary and Met Police chief to ensure none of the material taken from him is looked at until the case is resolved.
Bindmans said it had asked for assurances that "there will be no inspection, copying, disclosure, transfer, distribution or interference, in any way, with our client's data".
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Mr Miranda, whose partner Glenn Greenwald reported the Edward Snowden spying revelations, was held for the maximum nine hours after trying to change planes in London.
He was travelling to Brazil from Germany, where he had visited US filmmaker Laura Poitras who has been working on the Snowden NSA files with Mr Greenwald and The Guardian.
Scotland Yard insists the detention was "legally and procedurally sound" and the Home Office has now given police its full support.
A Home Office spokesman said: "The Government and the police have a duty to protect the public and our national security.
"If the police believe that an individual is in possession of highly sensitive stolen information that would help terrorism, then they should act and the law provides them with a framework to do that.
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"Those who oppose this sort of action need to think about what they are condoning. This is an ongoing police inquiry so we will not comment on the specifics."
Downing Street has also revealed it knew about the operation, but denies any political involvement.
It has prompted a storm of protest from civil rights campaigners and an apparent coalition split.
Lib Dem home affairs spokesman Julian Huppert called it unacceptable and "a clear abuse of terrorism powers".
A change.org petition calling for an urgent review of the laws, launched by Four Lions actor Adeel Akhtar who says he was held under similar legislation in the US in 2002, has already gathered 30,000 names.
Labour's Yvette Cooper called for an urgent investigation
"I'm not saying there shouldn't be a law in place to protect us from terrorist threats, but these laws that are being used, I don't think are being applied in the right way," he said.
Reporter Mr Greenwald called the detention a "profound attack on press freedoms and the newsgathering process".
He argued it was "clearly intended to send a message of intimidation to those of us who have been reporting on the NSA (National Security Agency) and GCHQ."
But the Metropolitan Police said: "The examination of a 28-year-old man under Schedule 7 of the Terrorism Act 2000 at Heathrow airport on Sunday … was subject to a detailed decision-making process.
"The procedure was reviewed throughout to ensure the examination was both necessary and proportionate. Our assessment is that the use of the power in this case was legally and procedurally sound."
It added: "Contrary to some reports the man was offered legal representation while under examination and a solicitor attended. No complaint has been received by the Metropolitan Police Service at this time."
White House officials have revealed the US was given advance notice by police about their plan but says America did not request the move and was not involved.
Mr Miranda claims UK officials were doing the bidding of the US by trying to force him to reveal passwords for his electronic devices.
He said: "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 a criminal or someone about to attack the UK … It was exhausting and frustrating, but I knew I wasn't doing anything wrong."
The row has deepened after Guardian editor Alan Rusbridger claimed agents from GCHQ were sent to The Guardian's head office to destroy hard drives containing Snowden data.
Mr Rusbridger, in a comment piece for his own paper, said he was told by phone a few weeks ago: "You've had your fun. Now we want the stuff back."
Shadow home secretary Yvette Cooper, in a new statement on Tuesday, said: "The Government needs to explain who authorised the use of terrorism legislation in this case and what the justification was."
The police and security services work hard to safeguard our national security, and they need powers to prevent terrorism.
"However for public confidence to be maintained it is important that the Government, police and security services are seen to abide by the rule of law, and to operate proportionately with proper checks and balances in place."
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