Clive Stafford Smith, Shaker Aamer's lawyer and Reprieve's director, told CBS his client has agreed to whatever conditions the British government want to put on him if he was released, "because he has nothing to hide".
He said: "What I have said and what Shaker has said for years is that if you have got an allegations against him, put up or shut up."
Commenting on why Aamer is still being held despite being cleared to leave, he said: "I think it is a fascinating question and I would love a little more transparency.
"I wish someone official would give me an explanation and they won't. No one will say why they won't let him go."
Earlier this year, Prime Minister David Cameron raised Shaker Aamer's case with President Barack Obama at a G8 summit and the British government has repeatedly stated that it wants him returned to the UK.
Despite having British residency and a British wife and four children living in Battersea, London, US authorities have repeatedly threatened to send him back to Saudi Arabia, his birthplace, against his wishes.
A spokesman for the Foreign Office said: "Mr Aamer's case remains a high priority for the UK government and we continue to make clear to the US that we want him released and returned to the UK as a matter of urgency.
He added: "Any decision regarding Mr Aamer's release ultimately remains in the hands of the United States government. We continue to monitor Mr Aamer's welfare through engagement with the US authorities."
The last British prisoner being held at Guantanamo Bay has spoken from his prison cell for the first time.
Shaker Aamer, who has been detained at the US military prison for 11 years told CBS's 60 Minutes show those being held there are treated like animals.
Shouting from his cell he said: "Tell the world the truth ... Please, we are tired. Either you leave us to die in peace - or either tell the world the truth. Open up the place. Let the world come and visit. Let the world hear what's happening.
"Please colonel, act with us like a human being, not like slaves."
He added: "You cannot walk even half a metre without being chained. Is that a human being? That's the treatment of an animal...It is very sad what is happening in this place."
Aamer, one of 164 prisoners at Guantanamo, has been held since 2002 without charge and is accused of being a close associate of Osama bin Laden, which he denies.
The United States intends to repatriate two detainees held at its controversial prison in Guantanamo Bay, Cuba, to Algeria, the White House said.
"We are taking this step in consultation with the Congress, and in a responsible manner that protects our national security," White House spokesman Jay Carney said in a statement.
"We continue to call on Congress to join us in supporting these efforts by lifting the current restrictions that significantly limit our ability to transfer detainees out of Guantanamo, even those who have been approved for transfer," he said.
Controversial comedian Frankie Boyle has gone on hunger strike in solidarity with a prisoner at Guantanamo Bay.Read the full story ›
Detainees at Guantanamo Bay will be forced feed during Ramadan hours, the Miami Herald is reporting. Those on hunger strike at the prison will only be feed after sunset and before dawn.
Navy Captain Robert Durand said the detention centre had only fed Ramadan-observing detainees at night during Islam’s holy month for years, and this year would be no different - even with the majority of captives on hunger strike.
He told the Miami Herald: "We understand that observing the daytime fast and taking nothing by mouth or vein is an essential component of Muslim observance of Ramadan.
"And for those detainees on hunger strike we will ensure that our preservation of life through enteral feeding does not violate the tenets of their faith.”
The number of inmates on hunger strike inside America's Guantanamo Bay military detention centre has now risen above 100.Read the full story ›
Comedian Frankie Boyle has today announced a defamation suit against MI5 and MI6 on behalf of Shaker Aamer, the last remaining British resident held in Guantanamo Bay.
Boyle joined legal action charity Reprieve in London this morning to confirm the launch of the captive's legal action.
Mr Aamer, who was rendered to Guantanamo in 2002, alleges that the intelligence agencies have told lies about him, claiming he holds links with al Qaeda, which prevent his release despite politicians in both the UK and US requesting he be freed.
Ahead of the announcement Boyle tweeted: "Shaker Aamer, with a British family and a kid he's never seen, (is) just trying to get home."
James Connell, a lawyer for Ali Abd al-Aziz Ali, one of the five men accused of plotting the 9/11 attacks has been speaking at a press conference today.
He addressed what he said was his client's participation in "peaceful resistance to an unjust system" on the first day of the military tribunal at Guantanamo Bay on Saturday.
He also said that the men has endured years of "inhumane treatment and torture" that has had serious long-term effects and will ultimately infect every aspect of the military commission tribunal.