The Home Secretary has revealed she was briefed in advance of the possible detention of David Miranda at Heathrow Airport.
But Theresa May said that it was not for her to tell the police who they should or should not stop at ports, or who they should arrest.
She said: “If it is believed that somebody has in their possession highly sensitive stolen information which could help terrorists, which could lead to a loss of lives, then it is right that the police act and that is what the law enables them to do.”
The Home Secretary, who has come under pressure to explain how much the Government knew about the planned detention of Mr Miranda after the White House said it had been given a “heads up”, said there were safeguards in place to make sure such arrests were conducted properly.
She told the BBC: “I was briefed in advance that there was a possibility of a port stop of the sort that took place.
“But we live in a country where those decisions as to whether to stop somebody or arrest somebody are not for me as Home Secretary, they are for the police to take, that’s absolutely right that they have their operational independence and long may that continue.”
Mr Miranda, partner of Guardian journalist Glenn Greenwald, was detained at Heathrow Airport under Schedule Seven 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 Seven 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 concern on the use of terror laws.
Lawyers for Mr Miranda have written to Mrs May and Britain’s most senior police officer challenging the legality of the decision to detain him for nine hours.
Law firm Bindmans said it has sent a letter to Mrs May and to Met Police Commissioner Sir Bernard Hogan-Howe asking for explanation of who requested, and for what purpose, police seized sensitive journalistic material from him.
They have also demanded assurances that none of the material will be disclosed or shared or used.
Criminal lawyer Kate Goold, from Bindmans, said: “We are most concerned about the disproportionate way in which these powers were used and the chilling effect this may have on freedom of expression.”
Shadow home secretary Yvette Cooper earlier demanded that Mrs May say whether she or the Prime Minister were informed about the decision to detain Mr Miranda.
Ms Cooper said: “If the police and Government had a different purpose in detaining David Miranda for nine hours they need to say so, and set out what their legal justification was.”
Downing Street has denied any political involvement in the decision to detain Mr Miranda.
Scotland Yard has defended the detention as “legally and procedurally sound” but David Anderson QC, the independent reviewer of terrorism legislation, has asked for an official briefing on the arrest, which he described as an “unusual case”.