On the day he marks 10 years locked inside the world's most notorious prison without having been charged with an offence, the last UK resident in Guantanamo Bay pleads with his captors: "Please torture me in the old way ... Here they destroy people mentally and physically without leaving marks."

Speaking from his cell through letters and comments published for the first time in The Independent today, Shaker Aamer, who has never stood trial, reveals the torment of his captivity and removal from his family. Yesterday a senior British source close to the talks admitted that Mr Aamer's detention was "unconscionable". His plight was raised most recently with the US Secretary of State Hillary Clinton last week.

Fears are growing for the welfare of Mr Aamer, from south London, who is now 45 and has a wife and four children. He has never met his youngest son. His lawyers are particularly concerned by the deterioration of his mental and physical state, which Mr Aamer describes vividly in his letters. He has lost 40 per cent of his body weight and is suffering from health problems, aggravated by long periods in solitary confinement.

The Independent has seen dozens of handwritten letters from Mr Aamer to his wife and family and today publishes a selection of extracts. Heavily censored and containing scrawled drawings to entertain his children, they paint a portrait of his time in Guantanamo. On 19 August 2002, he writes: "You won't believe me, my hand is killing me from writing and also my back. I am getting old. I just became 41... but physically I'm 50. I got arthritis, kidney problems, hearing problems, eye problems, my hair has fallen, my heart is aching."

On 9 August 2008, he says: "My sweetheart, yes I lost a lot of weight, yes I have a lot of sickness, yes I got short sight, yes my bones are aching, yes I got white hair, yes I got old, but ... my heart is still young, my mind still strong, stronger than ever."

One of his lawyers, Cori Crider, who visited Mr Aamer in Guantanamo last week, said: "Shaker has dropped to perhaps 150lb [68kg], his face bears the marks of suffering, and while he has a nigh-irrepressible spirit, the authorities seem determined to grind him down to nothing." Clive Stafford Smith, another of his lawyers, said his client had been reading 1984 by George Orwell. "You must read this book because you need to understand what is happening here in Guantanamo," Mr Aamer told him during a visit late last year, the notes of which were declassified two days ago.

Captured in Afghanistan in December 2001 and held in Kandahar and Bagram before being transferred to Guantanamo Bay on 13 February 2002, Mr Aamer claims he has been tortured over a number of years. He also alleges he was tortured in the presence of a British MI6 officer in Afghanistan. Mr Stafford Smith said Mr Aamer began his latest spell in isolation on 15 July last year.

A British resident, born in Saudi Arabia, Mr Aamer had indefinite leave to remain in the UK when he was reportedly sold to the US in 2001 by Afghan villagers for $5,000. He claims he was helping to build a school. The US claims he was fighting with the Taliban.

Mr Aamer is the only one of the Guantanamo detainees to allege he was tortured while a British Secret Intelligence Service agent looked on. He claims "John" was present in Afghanistan when his head was smashed against a wall. He also claims he was visited by agents three times in Guantanamo between 2002 and 2005 – by which time the use of torture was widespread knowledge.

It has been reported that Scotland Yard detectives are to fly to Guantanamo to interview Mr Aamer, but the UK Government has always denied complicity in torture. Mr Aamer's imprisonment was raised by the Foreign Secretary William Hague during talks with Hillary Clinton last December. A British source said the upcoming US election made the prospect of a quick release unlikely, but told The Independent the UK was committed to his release. "The UK takes the view his detention is unconscionable," he said. "We are conscious of the US political process, but 10 years is a very long time without charge." Another source familiar with the case reported a conversation with a senior US diplomat who expressed a desire to see Guantanamo shut down and Mr Aamer returned "because it was a source of embarrassment" and "a running sore that compromised the diplomatic mission". A Foreign Office spokesman said: "The Government remains committed to securing Mr Aamer's release."

Timeline: a decade without justice

December 1968: Born in Medina, Saudi Arabia.

1996: Moves to London and works as an Arabic translator for a firm of solicitors. Marries a British citizen and is eventually granted residency.

June 2001: Goes to Kabul to volunteer for an Islamic charity.

October 2001: US invasion of Afghanistan begins.

November 2001: Captured by the Northern Alliance and eventually handed to the Americans. Claims subsequently he was badly physically abused at Bagram, in the presence of a British intelligence officer.

February 2002: Brought to Guantanamo Bay, but is not charged with a crime, a situation which has not changed.

September 2005: Organises hunger strike among inmates and is placed in solitary confinement.

June 2006: Claims he was beaten for hours and asphyxiated during an interrogation on the same day three other Guantanamo inmates died.

June 2007: Cleared for release when the Bush administration acknowledges it has no evidence against him but remains interned.

February 2009: Becomes the last British citizen or resident at the camp when Binyam Mohamed is repatriated.

January 2012: The Independent reveals the UK Government has spent £274,345 fighting Aamer in court, including preventing his lawyers viewing evidence that may prove his innocence and end more than a decade in US custody.

From Camp X-ray... Shaker Aamer's letters home

19 August 2002

"I just became 41... but physically I'm 50. I got arthritis, kidney problems, hearing problems, eye problems, my hair has fallen, my heart is aching."

9 March 2005

"You are the soul of my life. You are the best of my heart. You are the light of my eyes. You are the oxygen in my lungs, you are the sun on my back, the sweetest taste of my mouth you are everything you are everything I need to live, to love, to be... Do you know how much you are important for my life. If you break I will break, if you become weak I will become weak and if you go I will go. You are my soul twin. I need you to be strong."

9 August 2008