Lawyers for a British resident imprisoned at Guantanamo Bay today made a public plea for Jack Straw to secure his release.

Shaker Aamer, a Saudi who is married to a British woman and has four young British children, has been held at the United States base on Cuba for three years. He is currently in solitary confinement.

He has indefinite leave to remain in the UK and was in the process of applying for British citizenship when he was detained by the US, said his lawyers.

Natalia Garcia, his UK solicitor, and Clive Stafford Smith, his US counsel, were to deliver a letter to the Foreign Secretary today saying that the Government had the "strongest moral obligation" to work for his release.

"His wife, Zinnira, and his four children, are all British citizens and we clearly have both legal and moral obligations to them," the letter said.

"He has been held by the US now for three years and the prospect of this continuing is a terrible weight upon his family. His wife has unfortunately faced mental health problems because of the pressures placed on her."

Saeed Ahmed Siddique, Zinnira's father, said that Mr Aamer’s detention had caused terrible suffering to his daughter, who lives in London with her children aged three to seven.

"She is living as a widow," he told BBC Radio 4’s Today programme. Her children are like orphans. Always they ask ‘When is my father coming?’

"She has lost her balance mentally. She was in hospital, and she was knocking on every door saying ‘This is not a hospital, this is Guantanamo Bay and I am looking for my husband’.

"If there is any case against him, he should face it. If there is not, he should be released. He can go through the legal process, but he must come here."

The lawyers said previous letters to Mr Straw, sent three weeks ago, had gone unanswered.

The detainee is a Saudi national and would face imprisonment, torture or even execution were he to be returned to his homeland, the lawyers claimed.

"The best information that we have been able to secure from knowledgeable sources suggests 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," said the letter.

"The best his British children would then look forward to would be a decade without their father. However, he is likely to face harsher treatment, even death."

Mr Aamer is one of five Guantanamo detainees recognised as British residents, although Mr Stafford Smith said there were at least two more.

All nine British nationals have now been released from the base and returned to Britain. The final four were released last month. None has faced criminal charges.

Speaking in Parliament last month, Mr Straw said there was nothing Britain could do for individuals in the position of Mr Aamer.

"We are not able to make representations on behalf of people held who might be resident here but are not our nationals," he told MPs.

But Mr Stafford Smith told Today: "The Government’s line is just smoke and mirrors.

"When they say they can’t intervene on a consular level, maybe that is true, but they can certainly intervene on many other levels.

"What I am asking the British Government to do is take the simple position that the father and husband of British citizens should have a right to be in this country."

A spokesman for the Saudi embassy in London said the kingdom was seeking Mr Aamer’s release and denied he would face persecution or be refused leave to return to the UK if freed. Ali Al-Khani told Today: "We are trying hard to secure his release, along with the others. Why would we torture him or persecute him?

"We will talk to the individual, as we always talk to people who were allegedly involved in terrorist actions. But five individuals have been released and are reunited with their families and they haven’t been persecuted or tortured."

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