Shaker Aamer, pictured with one of his daughters, has been behind bars in Guantanamo for 13 years

The U.S. is refusing to release Britain’s last Guantanamo Bay detainee because it fears he will expose shocking levels of brutality inside the notorious prison camp, a senior MP has claimed.

Andrew Slaughter, a Labour justice spokesman, said there were mounting suspicions that the American authorities wanted to keep Shaker Aamer behind bars to guarantee his silence.

The 48-year-old terror suspect, who has spent 13 years in captivity without charge or trial, claims to have witnessed US guards torturing and abusing fellow inmates at the camp in Cuba.

The Mail has also told how he claims to have been beaten 315 times.

Despite the latest salvo by British MPs against the White House, Mr Aamer’s lawyer yesterday expressed hope that his client could be freed ‘within weeks’.

Clive Stafford Smith said several US government officials had told him the Londoner, cleared for release by Presidents George W Bush and Barack Obama, could be sent home as early as next month.

In a letter released through his lawyer, Mr Aamer thanked his supporters. ‘I was shocked to see how much these MPs really care, really engage and ask the smart questions,’ he said.

‘I love these good people, not only for asking to release me, but for their effort to close Guantanamo, and how much they are against it.’

Labour MP Mr Slaughter travelled to the US last week with senior Tory MP David Davis, Tory former international development secretary Andrew Mitchell and Labour MP Jeremy Corbyn to urge the White House to free the father of four.

He said senators they met, including Republican former presidential hopeful John McCain, and Democrats Pat Leahy and Dianne Feinstein, were ‘puzzled’ by Mr Aamer’s ongoing detention.

Saudi-born Mr Aamer moved to London almost 20 years ago and married, securing leave to remain in Britain. In 2001 he was detained in Afghanistan while doing peaceful voluntary work for an Islamic charity, his supporters say.

He was handed to the US military for money and tortured at a secret ‘black site’ prison. US intelligence claimed he financed Al Qaeda and was a key aide of Osama Bin Laden, an accusation that he vehemently denied. He has been cleared for release, but US officials are determined that he should be sent to Saudi Arabia, even though there are serious concerns he will face torture there. His family still lives in South London.

Mr Slaughter said: ‘Shaker is articulate, intelligent and fluent in English and has been one of the organisers of opposition to the Guantanamo regime, including hunger strikes. He has witnessed some of the worst abuses at the base.

Despite mounting calls for him to be released - including from high profile MPs like Jeremy Corybn, Andrew Slaughter and Caroline Lucus - he is still trapped in the prison, where he is kept without charge

‘One reason submitted to us for them not wanting him to come to Britain was not because he would be a security risk but because they feared he would speak out about his treatment in Guantanamo, which would be embarrassing.’

Campaigners have branded the scandal of Mr Aamer continuing to languish in the naval base in Cuba as an ‘affront to civilised values’.

David Cameron was told in January by President Barack Obama that Mr Aamer’s case would be ‘prioritised’.

Mr Stafford Smith, director of human rights charity Reprieve, told the BBC that Mr Aamer could be released ‘within weeks’.

He said: ‘I have heard from various sources, which are very reliable I hope, that he is to be released in June. But of course we’ve had promises before and the worst thing one can do is, both for Shaker and for his wife and children, to promise something that may not happen. But there’s no good reason why it wouldn’t happen.’

Andrew Slaughter: Shaker was cleared for release by Bush in 2007 and Obama in 2009 - so why was he still in detention?

The Labour justice spokesman, who travelled to the States last year to argue on Aamer's behalf, questions why the father-of-four has not been released despite repeated promises

Imagine that a US citizen had been imprisoned, mainly in solitary confinement, without charge or trial for 13 years by the UK Government, and avoided due process by being kept out of the jurisdiction at a remote military encampment.

You would have to imagine because in real life this could not happen. The US would have found this treatment of its citizen by a close ally intolerable and ratcheted up the pressure until they were returned home.

But this is exactly the situation in which Shaker Aamer finds himself. He was rendered to Guantanamo Bay on 14 February 2002, having been sold to US forces in Afghan soldiers interested in the bounty offered for foreigners in Afghanistan post-9/11 but not in what he was doing there.

Shaker maintains he moved his family there to work for a charity promoting girls’ education. His wife and four children – the youngest of whom was born after his abduction - are all British citizens and he is a permanent resident.

Andrew Slaughter, second left, travelled to America last year to try and get Aamer released

It has taken time for Shaker’s case to achieve cause celebre status even in the UK. Even now his case is often overlooked in the US, despite forceful lobbying from all quarters and opinions here, not least from the Prime Minister who asked for his transfer to the UK to be a priority when he met President Obama in January.

So the formidable Save Shaker campaign resolved immediately following the election to send its own delegation to Washington to meet the main opinion formers there, and raised the cash to do so be combination of crowdfunding and collections in Trafalgar Square in front of a mock-up of a Guantanamo cell.

That is how I found myself in Washington last week meeting John McCain, State and Defense Department officials and leading Democratic Senators who have the President’s ear.

To show how broad the political opinion in support of Shaker’s release has grown my travelling companions were MPs David Davis, Jeremy Corbyn and Andrew Mitchell. All are members of the Shaker Aamer All-Party Group, ably chaired by John McDonnell MP, but have precious little else in common.

Aamer has been cleared for release by both Presidents Bush, in 2007, and Obama, in 2009

What did we think we could achieve that the PM, in raising the issue directly with the President, could not? And to answer the cynics’ question: we were all persuaded in meetings with foreign office officials here and our ambassador there that the UK Government is sincere and forceful in pressing the case for transfer.

Firstly, we wanted to find out in longer discussions what lay behind the refusal to release Shaker to the UK when countries such as Uruguay and Kazakhstan have in the last year been paid to take Guantanamo inmates who had no connection with those countries.

Given that Shaker was cleared for release by George Bush in 2007 and Obama in 2009, why was he still in detention? Shaker is articulate, intelligent and fluent in English and has been one of the organisers of opposition to the Guantanamo regime, including hunger strikes. He is also one of the longest-serving prisoners and has witnessed some of the worst abuses at the base.

He was cleared for transfer to Saudi Arabia. Why then not to the UK? Perhaps the US authorities think life will be too soft here and opportunities for talking about his experience greater. But if they genuinely suspect he is a risk and needs either de-radicalising or to be monitored or controlled, the UK has a much better track record than Saudi or many other countries. None of the 15 Guantanamo inmates so far returned here has been involved in seditious acts.

Seeking answers to our questions, we found them reflected back at us. None of the eminent and well-connected Democrat Senators – Feinstein, Leahy, Durbin and Manchin – could offer any explanation. However, this made them all enthusiastic to raise Shaker’s case with Obama and his Defense Secretary, in whose gift the transfer currently lies.

The second reason for our visit was to lobby directly John McCain. No friend of Obama and a hardliner on the so-called ‘war on terror’, he was nevertheless entirely sympathetic and I think persuaded. Whether this stems from his affection for the US-UK alliance or his own experience of torture and imprisonment by the Viet Cong, McCain promised to take up Shaker’s case. The importance of which is that it gives Obama cover against an increasingly dominant and shrill Republican Congress.

'For those brought up on a diet of US legal drama in which due process and constitutional rights were the dominant features of the justice system, what is happening at Guantanamo Bay seems beyond belief,' writes Slaughter

Thirdly, since Cameron’s visit in January, the House of Commons has debated Shaker’s detention and unanimously resolved that he should be immediately released and returned to his family in the UK. We have sent copies of the debate and motion to the Senators and the administration pointing out what a rare event this is. Now it is both the British Government and Parliament that is demanding his return.

Finally, the timing of or visit was designed to forestall the likely decision of Congress to take the decision on transfers out of the President’s hands and make it subject to such restrictions that it is likely that not one of the remaining 122 inmates will be eligible for release for the foreseeable future. We hope that Shaker’s transfer will be approved this summer, failing which his prospects look bleaker than ever.

For those brought up on a diet of US legal drama in which due process and constitutional rights were the dominant features of the justice system, what is happening at Guantanamo Bay seems beyond belief.