Shaker Abdurraheem Aamer has never seen his fourth child, Faris, and his wife has spent two months in a mental hospital with depression said to have been brought on by his situation.
Talking publicly for the first time, Mr Aamer's father-in-law asked the Government to help. He told The Independent "Shaker should have the right to come home and join his wife and children, who are British citizens. They are suffering without him."
Saeed Siddique, 62, a former religious minister who emigrated from India in 1982, said of his daughter, Zinnira: "She fell mentally ill late last year. She did not speak or eat. She did not sleep for a month. In December, she was admitted to a mental hospital for two months. The Government should help my son-in-law as they helped the British citizens who have been released from Guantanamo Bay."
Mr Aamer, 38, has been held at Guantanamo Bay since February 2002, after he was captured in Afghanistan. The American authorities have not disclosed the allegations against him.
A Saudi Arabian, Mr Aamer had lived in Britain since 1996. His wife and four children, who live in London, have British citizenship. Zinnira, 30, is now at home but is still unwell. Her children, aged between seven and two, are mostly cared for by Mr Siddique's wife, Aalyia, who is disabled with arthritis.
Like at least five other men who lived in Britain before being jailed at Guantanamo Bay, he has been refused government help because he was not travelling on a UK passport when arrested on suspicion of terrorism.
The last four British citizens held at Guantanamo Bay - Moazzam Begg, Feroz Abbasi, Martin Mubanga and Richard Belmar - returned home last month. Mr Aamer, who worked as an interpreter in London, was visited in Guantanamo Bay last month by the human rights lawyer, Clive Stafford Smith. "I was with him for three days," said Mr Stafford Smith.
"My impression was that he was really losing it. He was mentally unwell. He also seemed to have serious hearing difficulties, which he did not have before his capture."
At the time of Mr Stafford Smith's visit, Mr Aamer was held in solitary confinement in a cell two and a half metres long. Mr Stafford Smith is prevented from discussing what Mr Aamer said to him as this is considered a threat to US national security.
Mr Aamer is currently being held in Camp 5 at Guantanamo Bay, where captives are held in permanent solitary confinement. Mr Stafford Smith believes that a total of seven residents of Britain are being held in Cuba, but only five have been identified. The other four are: Bisher al-Rawi, Jamil al-Banna, Jamal Abdullah and Omar Deghayes.