The father-in-law of a UK resident held by the US in Cuba has urged the foreign secretary to secure his release.
The father of four was arrested in Afghanistan in 2002
Shaker Aamer, a Saudi national who is married to a Briton and is the father of four British children, has been held at Guantanamo Bay for three years.
Father-in-law Saeed Siddique said Mr Aamer's wife Zinnira had developed mental problems since his imprisonment.
A letter sent to Jack Straw said there was the "strongest moral obligation" to secure Mr Aamer's release.
The letter, written by Mr Aamer's UK solicitor Natalia Garcia and US counsel Clive Stafford Smith, said: "There is a British woman who may become a widow, and four young British children who face the prospect of being fatherless, if the government fails to act quickly.
"He has been held by the US now for three years, and the prospect of this continuing is a terrible weight on his family."
At a press conference in London on Tuesday, Mr Siddique said that Mr Aamer's 30-year-old wife had developed mental health problems three months ago and had been admitted to hospital.
Mr Siddique said: "She could not sleep day or night at all and was walking everywhere in the hospital knocking on every room.
"When the person came out asking why she was knocking on their door she said: 'I am looking for my husband.'"
Mr Stafford Smith added: "All we are asking the British government to do is give a British father the permission to come back to Britain.
"The Americans don't want to keep these guys at Guantanamo Bay - they are desperate to get rid of them."
Mr Aamer is believed to have been captured in January 2002 in Afghanistan, where he was working for a Saudi Arabian charity.
He had been living in the UK since 1996, was applying for citizenship and had indefinite leave to stay in Britain at the time of his capture.
The UK government has said it cannot make diplomatic representations to the US in Mr Aamer's case, or those of at least four other UK residents detained at Guantanamo Bay, because the men are not British citizens.
Saeed Siddique said Mr Aamer's wife was suffering mental problems
The nine British nationals who were held in Cuba have all been returned to the UK and freed without charge.
Mr Aamer has four children: daughter Johina, seven; five-year-old son Michel; son Saif, four; and son Faris, three on Monday, who Mr Aamer has never seen.
The family now live in London, but were resident in Birmingham at the time of Mr Aamer's capture.
Mr Aamer's lawyers said other letters sent to Mr Straw three weeks ago have gone unanswered.
In them they argued that Mr Aamer would face imprisonment and even torture if he was sent back to Saudi Arabia.
"The best information that we have been able to secure from knowledgeable sources suggest that he would likely face seven or eight years in prison for 'illegally' marrying a foreign citizen... without permission, and for leaving the country without government consent," they said.
But the Saudi government argues it is negotiating on Mr Aamer's behalf and that he would be allowed to return to the UK should he be freed.