The partner of the US journalist behind the Edward Snowden leaks launched legal action against Britain on Tuesday for holding him under anti-terror laws as the prime minister's office admitted it was kept informed about his detention.
David Miranda, a Brazilian national who has been working with his boyfriend Glenn Greenwald on the leaks, was held for almost nine hours on Sunday as he passed through London Heathrow Airport.
The Guardian meanwhile said the British government had forced it to destroy files or face a court battle over its publication of US security secrets leaked by Snowden, who has been granted temporary asylum by Russia.
"David Miranda is taking a civil action over his material and the way that he was treated," editor Alan Rusbridger, whose newspaper has worked with Greenwald and Snowden, told the BBC.
British police confiscated some of Miranda's electronic equipment including his mobile phone, laptop, camera, memory sticks, DVDs and games consoles, according to The Guardian.
"He wants that material back and he doesn't want it copied."
The detention of Miranda, 28, has caused an international outcry and sparked protests from Brazil. He was travelling home to Rio de Janeiro from Berlin at the time and was held in a Heathrow transit lounge.
The White House has said it received a "heads up" from the British government that police were about to arrest Miranda under anti-terror legislation but denied it had requested the action.
A source in Cameron's 10 Downing Street office denied any political involvement in Miranda's detention.
"The detention was an operational matter for the police. Number 10 was kept informed in the usual way," the Downing Street source told AFP on condition of anonymity.
Britain's Home Office interior ministry defended the decision to detain Miranda, saying police could detain an individual if they believed he possessed "highly sensitive stolen information that would help terrorism".
The legal firm acting for Miranda, Bindmans, said it was challenging the legality of Miranda's detention under Schedule 7 of Britain's Terrorism Act 2000, which applies to ports and airports, after being contacted on Sunday.
Bindmans said it had written to the Home Office saying it would go to court this week if it did not receive 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."
The wider questions of state secrecy and the law intensified when Rusbridger made his claim about being ordered to destroy some of the Guardian's Snowden files.
Writing in Tuesday's edition of the Guardian, Rusbridger said that two months ago he had been contacted by "a very senior government official claiming to represent the views of the prime minister".
The call led to two meetings in which "he demanded the return or destruction of all the material we were working on".
At the time, the paper was publishing a series of candid revelations about mass surveillance programmes conducted by the NSA and its British counterpart, GCHQ, after former NSA worker Snowden handed them thousands of documents.
Rusbridger claimed that in a call "from the centre of government", someone he does not identify told him: "You've had your fun. Now we want the stuff back."
The editor said the government threatened to use the courts to try and obtain the leaked documents if the paper did not destroy them themselves.
He said two GCHQ security experts oversaw "the destruction of hard drives in The Guardian's basement just to make sure there was nothing in the mangled bits of metal which could possibly be of any interest to passing Chinese agents."
Rusbridger did not explain why he had waited a month to reveal the destruction of the computer equipment.
A furious Greenwald said Monday that his boyfriend had been the subject of "a clear attempt at intimidation" and he vowed to train his sights on Britain and its intelligence services in future.
The Guardian was one of the original media partners of WikiLeaks and its founder Julian Assange, and helped to publish material from the mass of confidential cables leaked by US military worker Bradley Manning.