Britain's anti-terrorist legislation watchdog has called on the Home Office and Metropolitan police to explain why anti-terror laws were used to detain the partner of the Guardian journalist Glenn Greenwald for nine hours at Heathrow airport.

Amid mounting concern across the political spectrum over the treatment of David Miranda, David Anderson QC said the detention of Greenwald's partner on Sunday appeared to be "unusual".

Miranda said he was questioned by six agents on his "entire life" while held at Heathrow. Arriving at Rio de Janeiro airport on Monday, Miranda said: "I remained in a room. There were six different agents coming and going. They asked questions about my entire life, about everything. They took my computer, video game, mobile phone, my memory card. Everything."

In an interview with The World at One on BBC Radio 4, Anderson said that only 40 of the 60,000 to 70,000 people questioned under schedule 7 of the 2000 Terrorism Act are detained for more than six hours. "You can see what an unusual case this was if it is correct that Mr Miranda was held right up to nine-hour limit," Anderson said.

Miranda is the partner of Greenwald, who has written a series of stories for the Guardian revealing mass surveillance programmes by the NSA. He was returning to their home in Rio from Berlin when he was stopped at Heathrow and officials confiscated electronics equipment, including his mobile phone, laptop, camera, memory sticks, DVDs and games consoles.

During his trip to Berlin, Miranda visited 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 intervention by Anderson came as the shadow home secretary, Yvette Cooper, called for an urgent investigation into the use of schedule 7 of the Terrorism Act 2000 to detain Miranda. Cooper said ministers must find out whether anti-terror laws had been misused after detention caused "considerable consternation".

Cooper said public support for schedule 7 of the Terrorism Act could be undermined if there was a perception it was not being used for the right purposes. "Any suggestion that terror powers are being misused must be investigated and clarified urgently," she said. "The public support for these powers must not be endangered by a perception of misuse.

"The independent reviewer of terrorism legislation, David Anderson, has already warned of the importance of using schedule 7 of the Terrorism Act appropriately and proportionately. The purpose of schedule 7 is to determine whether or not someone is involved in or associated with terror activity.

"The Home Office and police need to explain rapidly how they can justify using that purpose under the terrorism legislation to detain David Miranda for nine hours. This has caused considerable consternation and swift answers are needed.

"The police and security agencies rightly work hard to protect national security and prevent terrorism. But public confidence in security powers depends on them being used proportionately within the law, and also on having independent checks and balances in place to prevent misuse."

The unease about the treatment of Miranda spread to Tory ranks as David Davis, the former shadow home secretary, warned that police appeared to have behaved in a heavy-handed manner.

Downing Street declined to answer questions about the treatment of Miranda on the grounds that it was an operational matter.

The Downing Street spokesman added that police would judge whether they had exercised their powers proportionately. He said: "The government takes all necessary steps to protect the public from individuals who pose a threat to national security. Schedule 7, which was used in this case, forms an essential part of the UK's border security arrangements. But it is for the police to decide when it is necessary and proportionate to use these powers."

But No 10's position was directly contradicted by Anderson, who said: "This is an important power. But the question of whether it was proportionately used in any given case is not ultimately for the police.

"The police, I'm sure, do their best. But at the end of the day there is the Independent Police Complaints Commission, which can look into the exercise of this power, there are the courts and there is my function. I report to the home secretary and to parliament every year on how this power is being used and whether it is being used properly."

Davis also dismissed the Downing Street position. He said: "This is absolutely not solely an operational matter for the police. This relates directly to press freedom and directly to our adherence to the rule of law. I'm afraid you cannot shove this one under the carpet on the basis of national security."

He added: "What did ministers know of this? Did they authorise it? Have they returned his computers, have they retained data from those computers and phones? There are a lot of questions to be answered as a matter of urgency.

"The truth is there is too much of a habit in Britain of using terrorism law as a catch-all … The 2000 act was not designed, and certainly not presented as a mechanism for trawling through people's private information when they passed through Heathrow between two other non-enemy countries."

Anderson, who has raised questions about schedule 7 of the 2000 terrorism act, said he hoped MPs would look carefully at the measure. The government is proposing, on the basis of a recommendation from Anderson, to reduce the maximum detention period from nine to six hours. The change is to be made through the antisocial behaviour crime and policing bill.

Anderson said: "At the moment anybody can be stopped under this power. There is no need for the police to believe they are a terrorist or to suspect they are a terrorist. The only reason they can talk to them is in order to determine whether they are a terrorist.

"It seems to me there is a question to be answered about whether it should be possible to detain somebody – to keep them for six hours, to download their mobile phone – without the need for any suspicion at all. I hope at least it is something parliament will look at."

Scotland Yard has refused to be drawn on why Miranda was stopped using powers that enable police officers to stop and question travellers at UK ports and airports.

"At 08:05 on Sunday 18 August, a 28-year-old man was detained at Heathrow airport under schedule 7 of the Terrorism Act 2000. He was not arrested. He was subsequently released at 17:00," it said in a brief statement.

Schedule 7, which applies only at airports, ports and border areas, controversially allows officers to stop, search, question and detain individuals. Miranda was held for nine hours, the maximum the law allows before officers must release or formally arrest the individual.