Shaker Aamer remembers the frantic knocking on the door, the voices screaming for him to get out. Outside, in the dark streets of Jalalabad, eastern Afghanistan, the soldiers stripped him of his belongings at gunpoint and marched away their latest prisoner.

It was November 2001 and Afghanistan was the focus of the furious US response to 9/11. The country that Aamer and his family had arrived in from London five months earlier had descended into chaos. The first US bombing waves had flattened the Kabul school where Aamer had taught English to the children of Arabic-speaking expatriates. Terrified, the Aamers fled east towards Pakistan.

Aamer had more reason than many to escape. Even when he was travelling with his pregnant wife and three children, Afghan rebels belonging to the anti-Taliban Northern Alliance, suspicious of all Arabs in the country, were likely to consider him a natural enemy.

He recounts how, after he had finally been caught and his family were allowed to go, he was driven into the countryside at night, expecting to be executed. Instead, he recalls the throb of a helicopter and friendly accents. He remembers exhaling with relief. "Americans!" he thought. "I am saved!"

More than 11 years later, Shaker Aamer has yet to meet his youngest son, Faris, who was born three months after his capture. On the day Faris was born, 14 February 2002, Aamer was airlifted to Guantánamo Bay, the soon to be notorious US detention camp. The subsequent years have been spent inside what has been condemned as the "gulag of our times". They have included more than 1,000 nights in a windowless isolation cell.

His daughter, Johina, 15, who lives in Battersea, south London, told the Observer: "Try imagining being treated like a circus animal in a cage and being taken away from your home and everything you love. It's painful, isn't it? Well, my dad is going through this."

The US forces who picked Johina's father up in Afghanistan appear to have had no intention of allowing him to go free, transferring him to the notorious Bagram jail at the end of December 2001. There, Aamer says, he was starved, kept awake for nine days straight and chained in positions that made the slightest movement unbearable. He became emaciated. Delirious and desperate to cease his torture, Aamer says he confessed to whatever the Americans wanted.

Those confessions form part of leaked detainee assessment briefs compiled by the joint task force that runs Guantánamo Bay. Marked secret, the documents claim that while in London Aamer had been "assessed to be a key member of the UK-based al-Qaida network with multiple associations to senior al-Qaida members". These allegedly include Osama bin Laden himself, whom Aamer is alleged to have met within the Afghan cave complex of Tora Bora. It is also alleged that his family in London received a "monthly stipend" from al-Qaida. For good measure, Aamer is described as having admitted frequenting the once notorious Finsbury Park mosque and the Four Feathers mosque, described as "the home of radical imam Abu Qatada".

These allegations, which Aamer vehemently denies and which no one has ever been able to prove, help to explain why Aamer has spent thousands of days in detention, a stretch of incarceration that has led him, in despair, to embark on a life-threatening hunger strike. So far detainee US9SA-000239DP has endured 68 days without food, far beyond what is accepted as safe. Clive Stafford Smith, his British lawyer, concedes that for the first time Aamer, widely regarded as a robust and resourceful character, has started to raise the possibility that he might die inside Guantánamo Bay. He recently told Stafford Smith, who is director of the legal charity Reprieve, to brief his wife that he might not make it out alive after all.

Shaker Aamer was born on 12 December 1966 in the Saudi Arabian city of Medina. His parents divorced when he was a child, and Aamer never got on with his stepmother. Aged 17, he headed to America to live with family friends. The next few years were spent travelling throughout Europe, the Middle East and finally London. There he met and fell in love with Londoner Zin Siddique, whom he married in 1997. Also that year, their first child Johina was born, followed by Michael in 1999 and Saif the following year. Family photographs from this period show a proud father framed by smiling children. Zin recalls them being very happy, describing their time together as a "dream". Aamer is described as a hands-on father, helping out with domestic chores and changing nappies.

It is the child who has never met his father who is understood to have struggled most. Faris, 11, is reported to play obsessively with the presents bought years ago by his father in the search for a connection. "He loves playing with the toys that Shaker bought for my other children. They are very special for him," said Zin, who returned to Battersea after her husband was taken.

In London, Aamer had forged a career as an Arabic translator for new arrivals. His work with refugees would, in June 2001, prompt Aamer's ill-fated decision to take his family to Afghanistan to do voluntary work for an Islamic charity. "Shaker was there to help the poor in Afghanistan, but himself became the victim of injustice," said Zin.

Aamer's continuing incarceration is all the more mysterious, given that the Americans ruled almost six years ago that he could be freed from Guantánamo. In June 2007, he was officially cleared for release. A security assessment by the US government acknowledged it had no concrete evidence against him. Two years later, the Obama administration reiterated the lack of a case against him, underlining the fact that he could be released.

So why is Aamer the only one among the 16 detainees who possessed British citizenship and residency who is still being held in Guantánamo?

Officially, the British government insists it is dedicated to extracting the father of four, a position it has publicly adopted for the past six years. Last Tuesday, the Foreign Office's human rights report of 2012 reiterated that it was committed to secure Aamer's release and return. His case, it said, had been raised on multiple occasions, including direct pleas from the foreign secretary, William Hague.

However, Sadiq Khan, the shadow justice secretary, believes the efforts are less concerted than they ought to be. "My fear is that he's been forgotten. My worry is that the force of representations in recent years haven't been as aggressive as they should be. There's just no reason why he should still be there. My concern is: out of sight, out of mind."

But there may be darker motives at work. Aamer's lawyers increasingly fear his chances of being allowed home to London are actually diminishing. Reprieve say Aamer is alone among the 779 who have been detained in Guantánamo Bay in having purportedly been cleared for release, but to only one country – Saudi Arabia. Repatriation to Saudi Arabia would, they warn, see Aamer detained indefinitely, his access to media and his lawyers hugely curtailed. Aamer has repeatedly protested against the possibility of forced repatriation to Saudi Arabia.

According to Stafford Smith: "The sole reason for the US to send Shaker to Saudi Arabia is to have him silenced, most likely by sentencing him to a long imprisonment after a sham trial."

Why might powerful interests desire the silencing of Shaker Aamer? Stafford Smith points out that his case has an incendiary element: he is allegedly able to describe in detail how a UK intelligence agent was present while he was beaten. A British operative, he claims, was present as a US interrogator repeatedly smashed his head against a wall shortly before he was sent to Guantánamo. Described as articulate and highly intelligent, Aamer's allegations of British complicity in his torture and detention would undoubtedly reopen the vexed and fraught debate over British complicity in the darker side of America's "war on terror". Aamer has already announced he is suing MI5 and MI6 for defamation.

Stafford Smith, who has access to classified material from MI6 that he cannot share, even with Aamer, alleges the British security services are actively misleading their US counterparts to ensure he is never allowed to return to Britain to tell his story.

"They've gone around bad-mouthing Shaker, saying things that are simply false," he says. "Not only were they part of his abuse but they falsified evidence against him."

The fabricated evidence, alleges the lawyer, includes claims that Aamer was an al-Qaida member, that he ever met Bin Laden and that he was associated with mosques which served as an "attack planning and propaganda production base for al-Qaida".

There is perhaps also another, more personal, reason why Aamer has been singled out for special treatment. Corroborating accounts portray the detainee as a stubborn presence who regularly defies Guantánamo Bay authorities by standing up for the rights of fellow detainees. Regarded as a leader among the camp's inmates, Aamer is an unwelcome thorn in the side of the prison authorities and a man possibly judged as a forceful personality who would make a formidable witness were he ever returned to the UK.

It was Aamer who initiated the first hunger strike at Guantánamo in 2005 after military police beat up a prisoner while he was praying. It was Aamer with whom the authorities negotiated to end the strike which involved hundreds of prisoners. As punishment, he was placed in isolation for 360 days despite prison rules permitting isolation only for a period of 30 days.

During a recent visit to Guantánamo, Stafford Smith met a man who, despite his deteriorating health, was determined to stand up to the authorities. "The other day they told him to close the hole in the cell door that they push food through. But Shaker, despite being on hunger strike, refuses to shut the hole. So they push his food through and it stays there all day where he can smell it."

Fears are mounting that even Aamer's formidable resolve may have limits. Experts say the current hunger strike is different from previous ones. The hopelessness of indefinite detention, defined by permanent separation from loved ones and the futility of wanting a return to normal life, has a deleterious and profound effect on prisoners' wellbeing.

Stafford Smith said Aamer sounds increasingly weak. That is hardly surprising: Aamer's hunger strike is approaching day 70, significantly beyond the point that experts say leads to "irreversible cognitive impairment and psychological damage".

After day 40, they say, "the possibibility of death becomes an imminent risk". Among the ailments Aamer is suffering from are serious arthritis, asthma, prostate problems, kidney, and neck and severe backache. He is understood to have lost more than 17kg in recent weeks.

Certainly the hunger strike inside Guantánamo Bay is escalating. Of the 166 detainees still there, more than a third have been classified as hunger strikers, the definition being that they have missed nine consecutive meals.

The Guantánamo regime, too, is widely reported to have become more hardline. Aamer talks of a return to "Miller Time", a reference to the notorious General Miller who presided over a brutal regime at the prison in 2003. Worst is the mounting frequency of Forcible Cell Extractions (FCEs) – violent removal of individuals from cells by armed prison guards.

"Now whatever Shaker requests they FCE him. If he asks for a bottle of water they FCE him, it's barbaric. Now he is refusing his water just so he doesn't get beaten up. It's gratuitous torture," said Stafford Smith.

The situation is such that Aamer is starting to suspect the regime at Guantánamo Bay is trying to kill him through medical neglect. Simultaneously, the strain on his family is starting to mount. Johina, a secondary school pupil who last saw her dad when she was four, asks the public to imagine what it must be like: "Imagine being locked up for more than 10 years of your life and possibly more years to come while everyone sits there and does nothing about it."