Amnesty International has called for the last UK resident still detained in Guantanamo Bay to be given a date for his safe return.
Around 18,000 people have signed a petition organised by the international human rights organisation which calls for Shaker Aamer's release without delay if he is not charged and brought to a fair trial. He has been held there for the last 13 years despite never being charged and being cleared for release.
Last Friday David Cameron raised the case with President Barack Obama, who gave a public commitment to prioritise the matter, Amnesty said.
Amnesty Northern Ireland director Patrick Corrigan said: "Those words must be followed by actions.
"No more excuses, no more delays - Mr Aamer should be given a date for a safe return to his family."
Mr Aamer, whose family live in south London, has been held without charge or trial at Guantanamo since February 14 2002, and was "cleared for transfer" from Guantanamo in 2007, Amnesty said.
Mr Corrigan and Belfast-born consultant neurologist David Nicholl - who has championed the Guantanamo detainees - raised the case in a meeting at the US Consulate General in Belfast this morning.
Mr Nicholl has led a campaign by UK medics for the release of Mr Aamer, who is suffering from a range of medical problems, including post-traumatic stress disorder.
"As a medical professional, I am extremely concerned for the health of Shaker Aamer. Shaker's ongoing detention is having an appalling effect, not just on his health, but on those of his family, especially his wife - all of whom are British citizens.
"As an Amnesty member and human rights campaigner, I am appalled that anyone can be locked up for 13 years of their life without ever being charged with or tried for a criminal offence."
Originally from Saudi Arabia, the 46-year-old has permission to live in the UK indefinitely because his wife is a British national. When Mr Aamer was detained in 2001, he had three children and his wife was pregnant.
He was arrested in Afghanistan in 2001 by US authorities, who said he led a unit of fighters against Nato troops and had met Osama bin Laden.