The fate of the last British resident to be held in Guantanamo Bay is in the hands of political powerbrokers in London and Washington ahead of crucial elections, and has little to do with national security and justice, his lawyer has claimed.

With a General Election in the UK now widely expected on May 6, and mid-term elections in the US in November, the potential release of Shaker Aamer, who has now been held in the notorious Cuban prison camp for more than eight years without charge, could be politically explosive.

Mr Aamer has accused British Secret Service agents of being complicit in his torture, which, coupled with his intimate knowledge of events at Guantanamo, means he has the capacity to be a political embarrassment to both governments.

As political and legal rows over his detention deepens, politicians on both sides of the Atlantic have blamed eachother for delays in the case - leading his family, solicitors and MPs to question diplomatic efforts, the “special relationship”.

Mr Aamer, 42, was captured in 2001 in Afghanistan, has been held in Guantanamo since 2002 on suspicion of helping the Taliban, but, despite being cleared for release by a US review board in 2007, he remains under lock and key.

Last week at a meeting Mr Aamer’s family, Foreign Office (FO) Minister Ivan Lewis blamed delays on American politics.

His comments were revealed at a press conference following the meeting.

Mr Lewis said: “There is no doubt there is a slowdown in the US for domestic reasons. We are doing everything we can.”

He said it “appeared to be related to American politics” adding the slowdown in the closure of Guantanamo Bay had not helped.

But Brent Mickum, who represents Mr Aamer - a Saudi born father-of-four who had indefinite leave to remain in the UK - in the US, said: “All the noises I am getting from US officials is that the Brits are not pressing hard enough.”

Mr Mickum recently saw confidential FO files he believes reinforce claims Mr Aamer is innocent and had been tortured. He was only granted access to them after a lengthy court case in the UK, and cannot reveal their contents.

But, he said: “I can say this. His detention is purely political. It has nothing to do with justice and what he has done. It is more to do with what has been done to him. That is the basis for his detention.”

Mr Aamer’s release before a General Election in the UK would allow the Government to gain the trust of liberal and swing voters and the Muslim community.

One Government source said: “There are about 2m Muslim voters in the country so if there is any movement it would obviously help, but it is very sensitive.”

But US politics could play its part too.

America is in the middle of a slow economic recovery, with about 10 per cent unemployment, and November’s elections are set to be tight - evidenced by the Democrat’s losing the apparently safe seat of Massachusetts in January.

“What this is, is a PR problem for both of them” Mr Mickum said.

Battersea MP Martin Linton said Mr Aamer was being denied basic rights. “It is detention not only without trial but without explanation,” he said, “If it wasn’t America he would have been brought back long ago or we would have intervened.”

Gareth Peirce, Mr Aamer’s solicitor in the UK, said following the meeting gave an insight into the “special relationship”. “After eight years of Shaker being in unlawful captivity it is frustrating in the extreme to meet a minister who is saying essentially that everything that can be done is being done and they are at a loss to think of anything else.

“It defies belief that we can’t influence our oldest ally to release someone held one of our own in unlawful captivity.

“There is no explanation for the extreme delays in his case and nothing suggests an urgent attempt to get him back.”

Mr Aamer’s wife, Zinnira Aamer, left the meeting despondent.

She said: “I think it is not enough to say it is a political issue. If it is then they should know what to do to get him back. He [Ivan Lewis] was asking me what I wanted him to do.”

Johina Aamer, who at 12 is the oldest of Mr Aamer’s four children, also met Mr Lewis. She said: “I want my dad back because I haven’t seen him in eight years. My smallest brother has never met his dad.”

Lawyers maintain the secret FO files prove Mr Aamer is innocent and show he was tortured.

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

“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.”