The family of the last Briton held in Guantanamo Bay, the extra-judicial US interrogation centre on Cuba, have spoken this weekend of their decade of torment as President Barack Obama appears set to perform "the biggest U-turn of his presidency" by keeping the prison open and signing indefinite detention without trial into law.

The family of Shaker Aamer – speaking as the 10th anniversary of the prison next week is expected to prompt worldwide protests – said his unlawful imprisonment had destroyed him and his family.

As a US presidential election year begins, President Obama is expected to renege on a 2009 promise to shut Guantanamo. Instead, he appears to be planning to keep it open permanently.

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Mr Aamer, who has four children with his British wife, the youngest of whom he has never met, was captured by the US in Afghanistan in December 2001 and claims he was tortured before and during his time in Guantanamo. He said he was helping to build a school when he was seized; the US said he was fighting with al-Qa'ida.

Despite never being charged with any offence and being cleared by a military commission in 2007, Mr Aamer, now 45, remains locked up and with little prospect of returning home soon to his family in London.

"In the 10 years Shaker has been there he has become old," Saeed Siddique, his father-in-law, said yesterday. "His hair has turned white and he is very ill.

"His children are growing now and it is difficult for them. The youngest one is nine and has never met his dad. He doesn't know why, and he tells his mum, 'My father doesn't love me because he never sees me'."

Mr Siddique, 69, added: "Since Shaker has gone, my daughter has become very ill. She has been treated for depression and hearing voices. When she is very bad, I have to look after her and the children for weeks. It is very hard for her and all the children.

"When he was captured, Shaker offered to let my daughter divorce him, but she said, 'No, I will wait for you'. She is still waiting."

Within days, President Obama is expected to sign the 2012 National Defence Authorisation Act, which gives money to the military annually. This year, it includes an amendment allowing the indefinite detention of anyone, including US citizens, who were "part of or substantially supported al-Qa'ida, the Taliban, or associated forces, under the law of war until the end of hostilities".

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Tom Parker, Amnesty International's US policy director on terrorism, counter-terrorism and human rights, said: "The implications for Guantanamo are quite simple: it is staying open now. This effectively enshrines indefinite detention in American law permanently, or at least as long as any group vaguely connected with al-Qa'ida continues, which basically means any Islamic group.

"That is huge. That is not what democratic governments do... you could call this the biggest U-turn of his [Barack Obama's] presidency."

Tom Malinowski, a Human Rights Watch spokesman, said: "It's something so radical that it would have been considered crazy had it been pushed by the Bush administration."

The mood was different on 22 January 2009, when, two days after his inauguration, President Obama fulfilled a pre-election pledge by signing his first executive order that Guantanamo should be shut within a year.

A report by Amnesty International has shown that, while more than 750 detainees have been sent to Guantanamo, only one has been tried under the federal court system.

The Amnesty report – Guantanamo: A Decade of Damage to Human Rights – highlights the duplicitous nature of America's stance on human rights internationally. It shows that at least 32 of the 171 detainees still held in Guantanamo today were transferred there before 22 January 2002. At least nine of those detainees were never charged with an offence.