The White House distanced itself from Britain's handling of the leaked NSA documents when representatives said it would be difficult to imagine the US authorities following the example of Whitehall in demanding the destruction of media hard drives.

As a former lord chancellor said the Metropolitan police had no legal right to detain the partner of a Guardian journalist at Heathrow airport under anti-terror laws, the White House suggested it would be inappropriate for US authorities to enter a media organisation's offices to oversee the destruction of hard drives.

The White House – which on Monday distanced Washington from the detention of David Miranda – intervened for the second time in 24 hours after the Guardian revealed that senior Whitehall figures had demanded the destruction or surrender of hard drives containing some of the secret files leaked by the US whistleblower Edward Snowden.

Alan Rusbridger, the Guardian editor, said that two GCHQ security experts oversaw the destruction of hard drives on 20 July in what he described as a "peculiarly pointless piece of symbolism". Rusbridger had told the authorities that the action would not prevent the Guardian reporting on the leaked US documents because Glenn Greenwald, the reporter who first broke the story, had a copy in Brazil, and a further copy was held in the US.

The White House responded with surprise to the report of the destruction. Asked at his daily briefing on Tuesday whether President Obama's administration would enter a US media company and destroy media hard drives – even to protect national security – the White House spokesman, Josh Earnest, said: "That's very difficult to imagine a scenario in which that would be appropriate."

The intervention by the White House came after the British government embarked on an aggressive offensive to justify the treatment at Heathrow of the partner of the Guardian journalist Greenwald.

Theresa May, the home secretary, confirmed that she was given advance notice of Miranda's detention as she praised the police action on the grounds that he possessed sensitive documents that could help terrorists and "lead to a loss of lives".

But May received a setback when Lord Falconer of Thoroton, the former Labour lord chancellor who was involved in introducing the anti-terror legislation used to detain Miranda, said the police had no right to detain him under the Terrorism Act 2000. Miranda was held for nine hours at Heathrow on Sunday under schedule 7 of the act, which allows police to detain people at ports and airports even if they are not acting suspiciously.

Falconer, who helped introduce the act in the Lords before he became lord chancellor in 2003, told the Guardian: "I am very clear that this does not apply, either on its terms or in its spirit, to Mr Miranda."

The former close ally of Tony Blair said that schedule 7 of the act allows police to detain someone even when they have no grounds for suspicion. Falconer added: "What schedule 7 allows an examining officer to do is to question somebody in order to determine whether he is somebody who is preparing, instigating or commissioning terrorism. Plainly Mr Miranda is not such a person."

The former Conservative prisons minister Crispin Blunt told Channel 4 News: "Using terrorism powers for something that doesn't appear to be a terrorism issue brings the whole remit of the laws passed by parliament to address terrorism into disrepute." But May praised the police action as she and Downing Street acknowledged they were given advance notice of the detention. May 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 or not 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. Long may that continue."

The home secretary, whose officials had initially declined to comment on the issue on the grounds that it was an operational matter, said it was right for the police to act because of the sensitive nature of documents in Miranda's possession. May added: "I think it is right, given that it is the first duty of the government to protect the public, that if the police believe 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. That is what the law enables them to do. But of course the law also has safeguards within it and we have an independent reviewer who, as David Anderson has already said, he will be looking into this case to ensure it was conducted properly."

Downing Street confirmed that the PM was also informed. "We were kept abreast in the usual way," a No 10 source said. "We do not direct police investigations."

The double confirmation, which followed a statement from the White House on Monday that it was given a "heads up" about the detention, marked an abrupt change of tactics by the government. Officials had declined to answer questions about the affair on the grounds that it was an operational police matter.

The government switched its response from it being an operational police matter after the Guardian disclosed GCHQ's role in overseeing the destruction of the hard disks in a basement of the newspaper's London office. A few hours before the White House statement, Rusbridger said it would be impossible to imagine a similar demand to destroy hard drives in the US.

He told the BBC News channel: "The British government has moved against the Guardian in a way that would be simply undoable in America. America has the first amendment and it has no prior restraint … The British government explicitly threatened prior restraint against the Guardian – ie that they would go to the courts to injunct us and to cede the material which would have the effect of preventing us from writing about it."

Rusbridger added in an interview with The World at One on BBC Radio 4: "It was quite explicit. We had to destroy it or give it back to them."

Rusbridger launched a strong defence of the Guardian's decision to comply with the request to destroy the hard drives after Index on Censorship described the action as "very disturbing". He told Channel 4 News: "Rather than return the material to the government I said we would destroy it in the knowledge that we already had copies in Brazil and in America. It seemed to be our duty to this material and to the public is to go on reporting. If we had waited for the courts to come in, judges would have been in control of that information."

Former shadow home secretary David Davis said No 10's confirmation that David Cameron was given notice of the detention of Miranda meant that ministers had, in effect, approved of his treatment. Davis told The World at One: "They didn't direct it, nobody is suggesting they directed it. But they approved it by implication. If the home secretary is told this is going to happen and she doesn't intervene then she is approving it."

May told the BBC: "No. We have a very clear divide in this country – and I think that is absolutely right – between the operational independence of the police and the policy work of politicians. I, as home secretary, do not tell the police who they should or should not stop at ports or who they should or should not arrest … I am pleased we live in a country where there is that separation."

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, 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.