The British government has defended the detention of a journalist's partner for nine hours at Heathrow Airport, saying authorities "have a duty to protect the public and our national security".
Police used an anti-terrorism law to detain David Miranda, the partner of Guardian newspaper journalist Glenn Greenwald, at London's main airport for nine hours on Sunday.
Miranda's electronic equipment was also confiscated, and he was questioned about both his links to the Guardian and his private life.
Greenwald has published stories about US and British surveillance programmes based on documents leaked by former National Security Agency contractor Edward Snowden.
The Home Office said on Tuesday that police were right to stop people suspected of possessing "highly sensitive stolen information that would help terrorism".
Britain's opposition Labour Party is demanding to know whether the government knew in advance of the police decision to detain Miranda.
The Guardian, meanwhile, announced on Tuesday that Miranda was challenging the Home Office's legal authority to detain him, and that the paper was supporting that legal action.
Greenwald has said that he will not be deterred from further reporting after the British government's action.
He said on Monday that the UK would regret the detention of David Miranda, which he described as an attempt to intimidate him for publishing documents leaked by Snowden.
Asked if Miranda's detention would deter future reporting, Greenwald said: "Absolutely not. If anything, it will do the opposite. It will embolden me: I have many more documents to report on, including ones about the UK, where I'll now focus more. I will be more aggressive, not less, in reporting.
"I will be far more aggressive in my reporting from now. I have many documents on England's spy system. I think they will regret what they have done."
Brazil's government has also complained about Miranda's detention as unjustified.
Miranda said six British agents questioned him continuously in a room at Heathrow airport. He was freed and his passport was returned only when he started shouting in the airport lounge, he said.
Under schedule 7 of the Terrorism Act 2000, UK authorities are allowed to stop, search and detain passengers at rail, air and sea ports without probable cause, for up to nine hours. Only 40 of the 60,000 to 70,000 people questioned under schedule 7 have been held for more than six hours.
Last night when asked, White House spokesman Josh Earnest would not deny that the US had been given data held on Miranda's electronic devices. Asked to rule out the possibility, he replied: "I am not in a position to do that right now."
Earnest said that the US had been given a "heads up" by UK authorities that Miranda was to be detained, but had not requested his detention.
Greenwald met Snowden in June in Hong Kong, from where he published the first of many reports disclosing mass, warrantless surveillance of telephone and internet traffic by the National Security Agency. He has also revealed details of surveillance of billions of internet communications by the UK's GCHQ.
Snowden faces criminal charges in the US for his actions. He is currently living in Russia, which has rejected US calls for his return.