Campaigners and MPs will demand the release of Guantanamo inmate Shaker Aamer at an event in Parliament tonight.
British resident Mr Aamer has been held for over a decade without charge or trial at the US prison despite having been cleared for release in 2007.
David Cameron, William Hague and Nick Clegg have held informal talks with US Secretary of State Hillary Clinton and US Defence Secretary Leon Panetta to urge the US government to release Mr Aamer and return him to Britain.
But a Save Shaker Aamer Campaign spokeswoman said: "Informal dialogue has failed.
"It is time for the UK government to publicly condemn the US for the torture of a British resident in breach of all international laws.
"There must be steps the UK can take to bring Shaker Aamer back home."
The meeting will take place in Room 15 from 7pm to 9pm.