Moazzam Begg is kept like an animal in a cage, deprived of food, sunlight, proper clothing - a gross violation of human rights, his father told a press conference in London on Tuesday.
“If Moazzam has done something wrong, then bring him back to this country and put him behind bars here. What outside pressure are we going to put on America?"
Begg was living in Islamabad, Pakistan with his pregnant wife and their three children when he was "kidnapped" by American Special Forces and removed to Afghanistan, according to his lawyers.
It was only when the International Red Cross contacted Begg's father at his home in Birmingham in March 2002 that he realised what had happened to his son.
Begg, of slight build and asthmatic, was removed from Afghanistan to Cuba in February 2003 after a campaign to secure his release began to gather momentum, say his supporters.
Other commentators at the conference added the entire world continued to watch in disbelief at the lack of political action to end imprisonment of captives.
"It is absolutely extraordinary, the lack of sense of drive and urgency which seems to characterise the British effort at the moment," said Stephen Jakobi, head of Britain's Fair Trials Abroad pressure group.
"One can only suggest it is because they [the British government] have failed, they know they have failed and they are extremely reluctant to confess they have failed," Jakobi added.
"We feel that Blair and Bush are partners in crime.”
Dr Ghiyath al-Din Siddiqi,
head of Britain's Muslim parliament
Nine Britons captured by the US as part of its "war on terror" are being held in Cuba on suspicion of links to al-Qaida, the organisation blamed for the 11 September 2001 attacks on New York and Washington.
The captives are among more than 600 detained at the high-security Guantanamo Bay camp, some of them for the past 18 months – most without formal or specific charges or legal representation.
Dr Ghiyath al-Din Siddiqi, head of the self-styled Muslim Parliament of Great Britain, said: "We feel that Blair and Bush are partners in crime.”