British government claims Greenwald's partner David Miranda had 'highly sensitive stolen information that would help terrorism'

Home Office claims nine-hour detention of Guardian journalist's partner was justified to protect public from threat of terrorism

The government has embarked on an aggressive offensive to justify the detention of David Miranda by suggesting that the partner of the Guardian journalist Glenn Greenwald possessed "highly sensitive stolen information that would help terrorism".

Amid calls from across the political spectrum for a fuller explanation of the treatment of Miranda at Heathrow after a detailed statement from the White House, the Home Office made clear that his nine-hour detention was fully justified on the grounds that he was carrying leaked documents.

A Home Office spokesperson 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. Those who oppose this sort of action need to think about what they are condoning. This is an ongoing police inquiry so will not comment on the specifics."

The statement by the Home Office, including a challenge to critics to think about condoning the leaking of sensitive documents, marks a significant change in tone by the government.

Downing Street and the Home Office had declined to answer questions about the detention of Miranda on the grounds that it was an operational police matter.

But in the face of growing criticism across the political spectrum, the Home Office has decided to go on the offensive and offer wholehearted support for the police with some details of the operation that led to the detention of Miranda.

The suggestion that Miranda was in possession of stolen documents, following the White House disclosure that Britain gave the US authorities a "heads-up" before he was detained, suggests that the intelligence services had been monitoring his movements and communications in some detail between Berlin and London.

Miranda was stopped at Heathrow en route to Rio de Janeiro, where he lives with Greenwald, who has written a series of stories for the Guardian revealing mass surveillance programmes by the NSA.

He was returning to their home from Berlin when he was stopped at Heathrow under schedule 7 of the Terrorism Act 2000, allowing officials to take away his mobile phone, laptop, camera, memory sticks, DVDs and games consoles.

During his trip to Berlin, Miranda met Laura Poitras, the US film-maker who has been working with Greenwald and the Guardian. The Guardian paid for Miranda's flights. Miranda is not a Guardian employee but often assists Greenwald in his work.

The statement from the Home Office was issued shortly after Labour stepped up the pressure on the government to give a full explanation of the detention of Miranda by demanding a statement from the home secretary, Theresa May.

The shadow home secretary, Yvette Cooper, said: "If the White House knew about the decision to detain David Miranda at Heathrow, surely the home secretary knew too? It isn't good enough for the Home Office to dismiss this as a matter for the police. The White House have made clear it was 'a decision by the British government' and the police have said it was a 'detailed decision making process … reviewed throughout'.

"Given the sensitivity of this operation and the continued questions about the use of terrorism legislation in this case, Theresa May cannot simply refer this to the police."

The Liberal Democrat home affairs spokesman, Julian Huppert, said he was concerned about "political interference" in the police's use of anti-terror laws to hold Miranda. He said it looks like the Guardian is "being victimised for publishing stories" and called on the government to clarify exactly who knew about the decision to take Miranda into custody at Heathrow airport on Sunday.

 © Guardian News and Media 2013