Shaker Aamer, the last UK resident to be held in Guantanamo Bay, is undergoing medical tests ahead of being reunited with his children, BBC News reports.
His lawyers said he needed urgent care and mental assessments.
Mr Aamer, 48, is reported to be planning to bring legal proceedings against the UK government over its alleged complicity in his mistreatment.
Campaigners have called for an investigation into the alleged use of torture at the US military prison.
Mr Aamer, a Saudi national, is expected to be reunited with his children later and will finally meet the son who was born on the day he arrived at Cuba’s Guantanamo Bay.
The father-of-four is suffering from post-traumatic stress disorder, his lawyer Ramzi Kassem told the BBC, and will not speak or be seen in public for some time.
In letters sent to the BBC earlier this month, Mr Aamer said: “I need a full medical check-up, somewhere totally confidential where they will finally respect my privacy.
“You must consider 239 to be like an alien from Mars – 239 is my prison number here and they always call me by it.
“I don’t know how I will respond to anything else – like my name, Shaker. I have known nothing about the real world for more than 13 years.”
Mr Aamer was first held by US authorities in Afghanistan in 2001 shortly after the 9/11 attacks but maintains he was in the country doing charity work.
Since 2007 Mr Aamer, who claims he was tortured, has been cleared for release twice, by US presidents George W Bush and then Barack Obama.
His legal representative Clive Stafford Smith said: “We had a promise from the prime minister that there would be a fully independent inquiry into all of this torture – unfortunately that’s not happened yet.
“What he [Mr Aamer] does want is that the whole world should know what did happen so we can set in place rules so that British agents and, let’s hope, American agents don’t get involved in the torture business in the future,” he told BBC News.
Mr Aamer is believed to be in line to receive compensation after deals were made with previous detainees.
The prime minister’s spokeswoman said: “There was a settlement in relation to detainees in November 2010 which was subject to a legally binding confidentiality agreement.”