By Jo-Anna K. Burnett Follow @jo_annak
Officials from the Foreign Office said the UK will continue to press for the return of the last British citizen held in Guantanamo.
Alistair Burt, parliamentary under secretary of state at the Foreign and Commonwealth Office, said continued detention of Shaker Aamer by the US was unacceptable.
"The United Kingdom will argue that [Aamer’s] detention is wrong," he said.
In a telling moment, Burt appeared to let slip that he knows why US authorities are holding Aamer.
"I have a supposition, but it's not a detail I can go in to," the minister admitted.
Burt also revealed the detainee had been cleared for transfer to Saudi Arabia, although his lawyer – Clive Stafford Smith of the prisoners' rights group Reprieve – said Aamer was unaware that he was only cleared for the Middle Eastern country.
Aamer is a Saudi national, UK permanent resident and married to a British citizen. He has been detained in Guantanamo for more than 11 years.
Battersea MP Jane Ellison, who led the debate, admitted she did not know why Aamer was in Afghanistan at time of his detainment, but said his return had become more urgent due to the hunger strike he started in failing health in February.
Kate Allen, director of Amnesty International UK, said: "We need to see the UK government renewing its efforts to secure freedom for Shaker Aamer if he is not to be charged.
"The situation at Guantanamo is at crisis point."
An e-petition with more than 100,000 signatures initiated Aamer’s parliamentary debate. Aamer joined about 85 other detainees in the hunger strike according to reports from Reprieve and Amnesty International UK.
Guantanamo has held nearly 800 detainees since 2002. Six hundred have returned to their home countries; nine have died in the prison.