Secret files that could prove MI5 agents were present during the torture of a British resident held by the US in Guantanamo Bay have been seen by his lawyer, who said "nothing in those documents has changed my mind".

Brent Mickum said the files reinforced accusations Shaker Aamer, the last Guantanamo detainee to be recognised by the UK, was tortured in Afghanistan and confessions he made were obtained under duress.

On Saturday, the Save Shaker Aamer Campaign, which, along with and Mr Mickum, accuses the UK Government of not pressing for Mr Aamer's release, held a demonstration outside Downing Street to mark the eighth anniversary of his detention in the notorious camp.

Mr Mickum said his client was effectively being gagged to cover up “wrong doing” - claims strengthened by an article that said US government officials may have conspired to conceal evidence three Guantanamo detainees could have been murdered during interrogation, he said.

Harper’s Magazine said three deaths, which were presented as suicides by hanging, could have been suffocations.

It also suggested it may explain why the US is reluctant to release Mr Aamer, who has claimed he was part-suffocated while being tortured the same evening.

Mr Mickum, who said he had been denied basic access his client, who after years of solitary confinement has failing health, said the documents compelled the UK Government to act.

"Before I saw these documents I stated unequivocally that the British Government is complicit in so far as it was present during his torture. Nothing that I have seen in those documents changes my mind," Mr Mickum, Mr Aamer's lawyer in the US, said.

"Generally speaking, and without revealing any detail from the documents, my overall impression of the documents is that they are exculpatory in nature, period.

“The documents are helpful to Shaker's defence, in so far as they describe the general allegations against Shaker, which we are in a position to refute almost unequivocally, and they show that he was tortured.

"What I have seen confirms my position that there is no legitimate evidence against him and confirms my continuing belief that he is being held by the Americans to cover up my government's wrongdoing.

"And unfortunately I have to, at this point, believe the British are not really advocating strongly enough for their return.

"As long as he is in Guantanamo he can't talk and I can't talk. He is still being tortured down there. And if he ends up dying down there I have to say there is blood on the British hands.

The British can only sit back for so long and say 'we did everything we could'. He never posed a threat and he doesn't pose one now.

"What this is, is a PR problem for both of them. The British need to do the right thing."

Mr Aamer, a Saudi Arabian national who has indefinite leave to stay in the UK, was captured in 2001 in Afghanistan, where the US claims he was helping the Taliban.

He claims he was tortured in Guantanamo and during his detention in Bagram, Afghanistan, where, he alleges, Britain colluded in that torture by sending agents to interview him.

Mr Aamer - whose wife and four children, the youngest of whom he had never seen, live in south London - has never being charged with any offence.

The sources for the Harper’s article were former Guantanamo guards, one a sergeant.

Mr Mickum said: "I have no reason to doubt the information provided by the sergeant is true and what we know is that my client was subjected to being tortured by maybe seven individuals at the same time as three people were alleged to have committed suicide.”

He added he thought it "likely" Mr Aamer's torture was in the same “black site” area, Camp No, identified by the Harpers article.

“That account appears to square with what happened to my client ... they choked his airway and put a mask over him," he said.

Mr Mickum last met Mr Aamer in May 2009 but said he had since been denied access.

"Anyone who has been kept in the conditions he has been maintained in has to be suffering and labouring highly," he said.

"My client wants to meet with me and he wants to talk to me, he is not being allowed to meet me or not being allowed to talk to me.”

He last phoned his client in early January but was advised Mr Aamer did not want to speak.

"Based on my conversations with him when I was able to talk to him I just know he wants to meet with me and wants to talk to me. This is a convenient way to keep this story from coming out, that's all it is," he said.

Clive Stafford Smith, director of human rights group Reprieve, said the Harper’s report added to his belief the US government was afraid of what Mr Aamer may reveal.

He said: “This is merely confirmation, fairly stark confirmation, that the reason they wanted not to send him home to his family in England, but rather to send him to [his native] Saudi Arabia was simply to gag him.

“I have known Shaker for sometime, because he is so eloquent and outspoken about the injustices of Guantanamo he is very definitely viewed as a threat by the US. Not in the sense of being an extremist but in the sense of being someone who can rather eloquently criticise the nightmare that happened there.”

Mr Aamer’s case could prove a huge embarrassment for the UK Government, which last year tried to overturn a High Court decision which allowed Mr Aamer's lawyers access to classified files.

The case is also viewed as being important in forcing the Government to go public with security information, and strongly resonates with that of Ethiopian-born Binyam Mohamed - a UK resident and former Guantanamo detainee, released earlier this year, who alleges he was tortured while held by US agents before being taken to Guantanamo.

A spokesman for the Foreign Office said: “We requested the release and return of Shaker Aamer to the UK in August 2007. Ministers, including the Foreign Secretary, and senior officials have made it clear to the US Administration on a number of occasions that our request for Mr Aamer's return still stands. The allegations made in the US press relating to events in 2006 are a matter for the US Government. We have no information relating to the claims."