Shaker Aamer, the last British resident held at Guantanamo Bay, has been released, Foreign Secretary Philip Hammond has confirmed.
Mr Aamer, 46, is on a plane back to the UK after 13 years in detention at the US military prison in Cuba for 13 years.
Andy Worthington, co-director of the We Stand With Shaker campaign, said: "We're delighted to hear that his long and unacceptable ordeal has come to an end.
"We hope he won't be detained by the British authorities on his return and gets the psychological and medical care that he needs to be able to resume his life with his family in London.
Human rights charity Reprieve wrote on Twitter: "Looks like a plane has left Guantanamo Bay, bound for London."
Mr Aamer has said he was originally seized by bounty hunters while working as a charity worker in Afghanistan in 2001 shortly after the 9/11 attacks.
He was handed over to US forces and in February 2002 he was transferred to Guantanamo Bay and accused of aiding al Qaida.
During his time in captivity, his lawyers said he was subjected to torture, with beatings and sleep deprivation, and held in solitary confinement for 360 days. In 2005, he lost half his body weight during a hunger strike.
His family, MPs and actors Mark Rylance and Maxine Peake have also taken part in a 24-hour fast to show their support. Mr Aamer said he cried when he read about the protests.
He was described in US military files obtained by the WikiLeaks website as a "close associate of Osama bin Laden" who fought in the battle of Tora Bora. But in 2007 the allegations against him were dropped and he was cleared for release.
Despite a formal request by then-foreign secretary David Miliband, American authorities refused to allow him to go.
In letters, Mr Aamer said he was not sure if he would know how to respond to his name after being referred to as 239 - his prison number - for more than a decade.
Amnesty International UK director Kate Allen said:"After so many twists and turns in this appalling case, we won't really believe that Shaker Aamer is actually being returned to the UK until his plane touches down on British soil.
"We should remember what a terrible travesty of justice this case has been, and that having been held in intolerable circumstances for nearly 14 years Mr Aamer will need to time to readjust to his freedom."