A 20,000-strong petition will be presented to US President Barack Obama to urge the release of a British detainee at Guantanamo Bay who has been held at the camp for exactly 11 years.
Shaker Aamer, 44, was taken to the notorious US detention centre on February 14 2002 under suspicion of recruiting and financing terror group al Qaida.
Aamer has never been charged or tried with an offence and remains detained despite the US authorities officially approving him for transfer in 2009.
Through his lawyers, Aamer has alleged he was subjected to torture, including severe beatings and other ill-treatment, while held in secret US detention in Afghanistan and at Guantanamo.
Human rights organisation Amnesty International will take a petition to the US embassy in London to urge Mr Obama to negotiate his release if he is not to be charged or face trial, and clarify conditions under which he will be returned to the UK.
Amnesty International UK director Kate Allen said: "The continued detention of Shaker Aamer is totally baffling and the issue has become a running sore in UK-US relations. He faces no charges, was officially approved for transfer out of the camp years ago and the UK has said it will accept him back - so what is the hold-up?
"Given the time involved, the lengthy spells in solitary confinement and the torture allegedly used against him, Shaker's plight illustrates why Guantanamo has become such an affront to human rights.
"Guantanamo has always been a complete travesty of justice. It's time for President Obama to live up to his promise to close this icon of lawlessness and either bring the detainees before federal courts for fair trial on the US mainland, or safely release them."
Aamer's case is also the subject of a similar campaign by Amnesty activists in the US, where a 28,000-strong petition has been raised. He is one of 166 detainees still held at Guantanamo - out of 779 men taken to the facility in the last 11 years. The majority have been held for years without charge or criminal trial.
In total, seven detainees have been convicted by military commission, including five as a result of pre-trial agreements under which they pleaded guilty in return for the possibility of release from the base.