Media playback is unsupported on your device Media caption David Miranda told the BBC he felt very threatened during his detention

Home Secretary Theresa May has defended the police's use of anti-terror laws to hold and question David Miranda, the partner of a Guardian journalist.

She said it "was right" if police thought that Mr Miranda was holding information useful to terrorists.

But former Lord Chancellor Lord Falconer said it was not what the powers were meant for.

Mr Miranda is the partner of Glenn Greenwald, who has been reporting on US and British surveillance programmes.

He was held for nine hours at Heathrow Airport on Sunday and said his interrogators threatened he could go to prison if he did not co-operate.

Brazilian national Mr Miranda, 28, was detained under schedule 7 of the Terrorism Act 2000 as he travelled from Berlin to Rio de Janeiro, where he lives with Mr Greenwald.

The law allows police to hold someone for up to nine hours for questioning about whether they have been involved with acts of terrorism.

Mr Miranda's lawyers said he also had his mobile phone, laptop, DVDs and other items seized.

Law firm Bindmans said it had started legal proceedings to "protect the confidentiality of the sensitive journalistic material" seized.

It said the judicial review application due to be heard at London's High Court on Thursday aimed to secure a temporary restriction to stop the authorities using the material.

"If interim relief is not granted then the claimant is likely to suffer irremediable prejudice, as are the other journalistic sources whose confidential information is contained in the material seized by the defendants," Bindmans said in a statement.

'Not right'

If you know the person is not a terrorist, that power is not open to you Lord Falconer

Mrs May told BBC Radio 4's World At One programme: "If the police believe someone has in their possession highly sensitive stolen information that could help terrorists that could lead to a loss of life, it is right the police should act.

"I believe schedule 7 of this act enables police to do that."

But Lord Falconer, who helped to bring in the anti-terror legislation, said: "It does look like the wrong powers were used. Schedule 7 of the terrorism act is to be used... to discover whether somebody is a terrorist.

"If you know the person is not a terrorist, that power is not open to you."

He said the justification given by the home secretary "doesn't look right".

Lord Carlile, the government's former terrorism watchdog, said "we must know why the decision to use schedule 7 was made and if it was correct".

He said that if it was incorrect whoever made the decision to use the law must be held to account.

Separately, the police watchdog said it was dealing with 45 complaints about other cases involving the terror laws used to detain Mr Miranda.

However the Independent Police Complaints Commission said no complaint has been received about the stop at Heathrow.

Media playback is unsupported on your device Media caption Lord Falconer: "It does look like the wrong powers were used"

A poll by YouGov of more than 1,850 adults found that two-thirds (66%) backed the use of schedule 7 but nearly half (44%) believed the detention of Mr Miranda constituted an inappropriate use of the law.

Mr Greenwald has been covering stories based on leaks by Edward Snowden, a former contractor at the US National Security Agency (NSA) turned whistle-blower.

Mr Snowden leaked details of extensive internet and phone surveillance by American intelligence service, from which allegations about electronic snooping by British authorities arose. He is in Russia where he has been granted temporary asylum.

Earlier it emerged that Prime Minister David Cameron ordered Britain's top civil servant, Cabinet Secretary Jeremy Heywood, to contact the Guardian about material it had obtained from Mr Snowden earlier this year.

The discussions resulted in the newspaper destroying a number of computer hard drives last month, under the supervision of intelligence experts from GCHQ.

Media playback is unsupported on your device Media caption The BBC's Danny Shaw says David Miranda's lawyers will go to the High Court on Thursday

'Out of tune'

Senior members of the government defended Mr Cameron's decision.

Deputy Prime Minister Nick Clegg, said the approach by Sir Jeremy was "preferable" to taking legal action, while Mrs May said his involvement was an appropriate response to a possible threat to national security.

Foreign Secretary William Hague said: "The government clearly has a duty if information is held insecurely and could be damaging to our national security to try to make sure that it is recovered or destroyed. It's a very simple matter."

But Labour's Keith Vaz said the actions were "unprecedented" and called on the prime minister to make a "full statement" to Parliament on the day it returns after the summer break.

Russian Foreign Ministry spokesman Alexander Lukashevich was also critical.

"The measures taken by the British authorities towards the Guardian newspaper are out of tune with the British side's statements on commitments to universal standards of human rights, including the area of mass media, protecting the rights of journalists, and private life," he said.