The release of Shaker Aamer comes after a publicity campaign and at the request of Prime Minister David Cameron, who had urged President Barack Obama to resolve the case of the last prisoner at Guantanamo with significant ties to Britain.
"He needs, first, to be in a hospital, and then to be with his family," said Clive Stafford Smith, one of his lawyers.
His release, the 15th from Guantanamo this year, brings the detainee population there to 112, and comes as part of a renewed push by Obama to close the facility opened by his predecessor after the Sept. 11, 2001, attacks on the World Trade Center in New York.
Aamer, 48, had told his lawyers that he would seek a medical examination in Britain because of concerns about his health stemming in part from repeated hunger strikes while at Guantanamo.
He has received more media attention over the years than any other prisoners except the five who face trial by military commission for their alleged roles planning and providing support to 9/11 attacks.
Aamer was born in Saudi Arabia and remains a citizen, but wanted to return to London where he has four children, including a son he has never seen and a wife, who is the daughter of a prominent retired imam. Aamer worked as a translator for a law firm in London from 1994 to 2001.
He has said that he went to Afghanistan to help run a school for girls, and fled during the chaos following the U.S. invasion. He was captured by the Northern Alliance and turned over to the U.S. for a bounty. He was taken to Guantanamo in February 2002.
The U.S. Defense Department has disclosed that he was accused of significant links to terrorism. They said he shared an apartment in the late 1990s with Zacarias Moussaoui, who was convicted of taking part in the Sept. 11 conspiracy; had met with Richard Reid, who tried to blow up a U.S. passenger jet with explosives in his shoes; had undergone al-Qaida training in the use of explosives and missiles, and received a stipend from Osama bin Laden.
Those allegations and more were later found in a November 2007 detainee assessment obtained and published by Wikileaks that described him as a member of al-Qaida and a "close associate" of bin Laden.