Lawyers for the partner of the Guardian journalist Glenn Greenwald have said police officers who held and questioned him for nine hours at Heathrow airport under anti-terror laws did not provide him with any explanation for his detention.

They also said that David Miranda, whose native language is Portuguese, was not given an interpreter and that they refused his request for a pen to enable him take notes of the questions he was asked.

Miranda said he had been questioned by seven agents about his "entire life" and treated as if he were a criminal.

Now the law firm Bindmans, who were hired by the Guardian to deal with his detention on Sunday, have confirmed that they were not allowed access to Miranda until eight hours after he had been stopped in a transit area at Heathrow.

In a letter to the home secretary, Theresa May, and the Metropolitan police commissioner, Sir Bernard Hogan-Howe, threatening legal action over his "unlawful" detention, Bindmans said that officers stopped and detained Miranda at 8.05am on Sunday, shortly after he had begun changing flights from Berlin en route to to Rio de Janeiro, where he lives with Greenwald.

Once the Guardian became aware of his detention, multiple efforts were made to make contact with our client, the letter says. Bindman's lawyer, Gavin Kendall, went to Heathrow in person "as he was given no telephone access to our client".

He arrived at Heathrow just before 3.30pm and when he made contact with the police the only explanation given was that he was being detained under schedule 7 of the Terrorism Act 2000.

"At 4.05pm, Mr Kendall was finally granted access to our client, just one hour before the nine-hour statutory maximum detention power expired."

"Mr Kendall asked whether our client was being detained as a result of a suspected offence in the UK or on behalf of another state, country or government organisation abroad. He was told the police could not say and was not provided with any explanation for his detention.

"They refused to confirm what our client had been asked before his representative arrived, nor would they provide him with a record of what was discussed. Our client asked for a pen to write down the questions and this too had been refused," the letter reveals.

Miranda who is Brazilian was "not provided with an interpreter and found the whole experience most distressing," said Bindmans.

The officials confiscated his laptop, phone, two memory sticks, two DVDs, a Sony games console, a smartwatch and a hard drive, the letter revealed. "These items contain sensitive, confidential journalistic material and should not be detained," said Bindmans.

Miranda is now threatening legal action against the government and the Met if they do not return his belongings within seven days and give immediate undertakings that if any data on his electronic equipment inspected is not shared with any third party, domestic or foreign.

On Monday night, the Met issued a statement on his detention under schedule 7, saying their "assessment is that the use of the power in this case was legally and procedurally sound".

This echoes the experience of the US documentary film-maker Laura Poitrus, who has been working with Greenwald and whom Miranda visited in Berlin before his detention at Heathrow.

She said in a magazine article in the New York Times on Saturday that she has endured years of searches at airports because of her interest in national security, and on one occasion last year when she was refused a pen, was told that it could be used as a weapon. She then asked for a crayon and was refused that too.