British officials investigated al-Rawi's case after it emerged he had provided assistance to MI5, Britain's domestic spy agency.
George Brent Mickum IV, a US lawyer representing al-Rawi, said his client had agreed to work for the British spy service in exchange for his release.
Al-Rawi and Jamil el-Banna, another British resident, were alleged to have been associated with al-Qaeda through their connection with Abu Qatada, a London-based cleric.
The two were arrested in 2002 in Gambia while trying to return to Britain with electronic equipment that authorities described as "suspicious".
"[Al-Rawi] has been held in Guantanamo Bay in total isolation – lights on, 24 hours a day, in a six-foot by eight-foot cell"
Zachary Kaznetzelson, senior legal counsel
Zachary Kaznetzelson, a senior counsel from the US law firm representing al-Rawi, told Al Jazeera: "He was actually arrested in [London's] Gatwick airport. The device turned out to be a perfectly innocent battery charger.
"The police in London apologised and let him go but ... the British had sent information to the Americans that he was travelling with a suspicious device.
"The British never then corrected the initial information, to say the device was innocent. That was the basis for [al-Rawi's] original seizure."
Lawyers claim that after their arrests in Gambia, the men ended up in American custody.
Mickum said that the US Central Intelligence Agency (CIA) took them from Gambia on a "rendition flight" to Cairo, Egypt, where the plane re-fuelled.
They were then taken to a CIA facility in Afghanistan where they were interrogated as suspected terrorists.
Al-Rawi said in his statement that he and el-Banna were held in a CIA underground prison close to Kabul before being transferred to Guantanamo Bay.
Kaznetzelson said al-Rawi had been subjected to extreme treatment while in detention.
"[Al-Rawi] has been held in Guantanamo Bay in total isolation – lights on, 24 hours a day, in a six-foot by eight-foot cell," he said.
Al-Rawi said in his statement issued through Reprieve that he wanted to advocate for the remaining British citizens held in Guantanamo Bay.
"I hope everyone who believes in justice and the rule of law will join with me to work for the release of Jamil and the other British residents," he said.
Britain has secured the release of all nine of its citizens who were held at Guantanamo but says the release of nine others who were resident in Britain but not nationals is not it's responsibility.