Britain is facing intense pressure to give a detailed explanation of the decision to detain the partner of the Guardian journalist Glenn Greenwald after the White House confirmed that it was given a "heads up" before David Miranda was taken into custody for nine hours at Heathrow.

As the UK's anti-terrorism legislation watchdog called for an overhaul of the laws that allowed police to confiscate Miranda's electronic equipment, the US distanced itself by saying British authorities took the decision to detain him.

The intervention by the White House will put pressure on Downing Street, which declined to comment on the detention of Miranda on the grounds that it was a police operational matter and added that the Met would decide whether its officers had acted in a proportionate manner.

The No 10 position was immediately challenged by David Anderson QC, the independent reviewer of terrorism legislation, who described the detention as unusual and said decisions about proportionality were not ultimately for the police. He told Radio 4's The World at One: "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."

Last night, the Metropolitan police, which is due to meet with Anderson today, issued a statement in which it said the detention was "legally and procedurally sound". The statement said: "The procedure was reviewed throughout to ensure the examination was both necessary and proportionate. Our assessment is that the use of the power in this case was legally and procedurally sound."

It added: "As with any power, it is important that it is used appropriately and proportionately. There are a number of safeguards in place to ensure this happens."

Miranda, a Brazilian national who lives with Greenwald in Rio de Janeiro, described his nine hours' detention - the maximum time allowed under schedule 7 of the Terrorism Act 2000. He said: "They were threatening me all the time and saying I would be put in jail if I didn't co-operate. They treated me like I was a criminal or someone about to attack the UK ... It was exhausting and frustrating, but I knew I wasn't doing anything wrong."

The prospect of an investigation by the IPCC is likely to have been enhanced by the disclosure that the US authorities were given advanced notice of Miranda's detention after his name appeared on a passenger manifest. Miranda was detained at Heathrow on Sunday morning as he flew home from Berlin to Rio.

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.

Josh Earnest, the principal deputy White House press secretary, said at the daily briefing: "There was a heads up that was provided by the British government. This is something that we had an indication was likely to occur but it is not something that we requested. It was something that was done specifically by the British law enforcement officials. This is an independent British law enforcement decision that was made."

Earnest had earlier said: "This is a decision that was made by the British government without the involvement - and not at the request - of the United States government. It is as simple as that."

The White House spokesman confirmed that Britain alerted the US authorities after Miranda's name appeared on a passenger manifest of a flight from Berlin to Heathrow on Sunday morning. "I think that is an accurate interpretation of what a heads up is," Earnest said.

He would not rule out whether the US authorities had been passed any information from Miranda's electronic equipment seized at Heathrow, which included his phone, laptop, memory sticks, DVDs and games consoles. "I'm not in a position to do that right now," Earnest replied.

Miranda said he was questioned by six agents on his "entire life" while held at Heathrow. Arriving at Rio de Janeiro airport yesterday, Miranda said: "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 the World at One interview, Anderson said only 40 of the 60,000 to 70,000 people questioned under schedule 7 were 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.

The government is proposing, on the basis of a recommendation from Anderson, to reduce the maximum detention period from nine to six hours.

Anderson added: "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."

Keith Vaz, the Labour chair of the commons home affairs select committee, wrote to the Metropolitan police commissioner, Sir Bernard Hogan-Howe, to ask him who took the decision to detain Miranda. Vaz, who said his committee would examine the use of the law used as part of an inquiry into terrorism, said: "This is an extraordinary twist to an already complex story. It is right that the police have these powers but it is important that they are used appropriately."

The White House's explanation contrasted with the attitude of Downing Street, which declined to answer questions on the grounds that it was an operational matter. The prime minister's spokesman 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."

David Davis, the former shadow home secretary, said the Home Office should issue a detailed response in light of the White House disclosure. Davis told the Guardian: "I find it wholly implausible that the Metropolitan Police would have taken such a decision and somehow notify the White House but not notify the home secretary ... Therefore the government has to say what was going on here. We are a country where freedom of speech and freedom of movement are fundamental and that is never more true when you are talking about journalists holding the light up to the actions of government."

The Liberal Democrats indicated they would be looking for assurances that police had acted in a proportionate manner. A spokesperson said: "The independent reviewer of terrorism legislation has already asked for more information on this incident and we will wait to hear his conclusions."