Shaker Aamer, a Saudi national married to a Briton, was suspected by US officials of being an Islamist militant associated with al Qaeda but was never charged with any crime.
Television images from Biggin Hill airport south of London showed the arrival of a small white aircraft. A government spokesman said Aamer was back in Britain.
Britain said last month Aamer would be released, and foreign secretary Philip Hammond said earlier yesterday the former detainee was en route to Britain.
“Today is a day for welcoming him back and hoping that he is healthy and well and that can join his family at long last,” Kate Allen, UK director of Amnesty International, told Sky News.
In Washington, the Pentagon said it reviewed Aamer’s case and concluded he should be released.
“As a result of that review, which examined a number of factors, including security issues, Aamer was unanimously approved for transfer by the six departments and agencies comprising the task force,” said a Pentagon statement.
Aamer was cleared for release by US authorities in 2007 but not freed until now. During a debate in parliament earlier this year, some British lawmakers expressed concern that Aamer was still in Guantanamo because he “had seen too much” and could still pose a threat to America if released.
Rights group Reprieve said Aamer moved to Britain in 1996 and was in Afghanistan in 2001 doing voluntary work for an Islamic charity when he was captured by Afghan Northern Alliance forces and handed to the US military. He was transferred to Guantanamo when the prison camp opened in 2002.
Several British lawmakers long argued for Aamer’s release with John McDonnell, now the opposition Labour party’s finance spokesman, telling parliament earlier this year that Aamer had “endured harsh, and brutal and inhuman treatment”.
Allen said Britain should hold a “proper independent inquiry” into allegations including mistreatment.
Prime minister David Cameron’s spokeswoman said the government had no plans to detain Aamer but would do “anything necessary to ensure public safety”.